The problem is that it is not always true - not all issues are a matter of opinion, some are a matter of fact and in such cases you are not really entitled to a wrong opinion, well, any more than you are entitled to be insane.
Of course, saying this (even without saying "insane") will get the reply "well, that is just your opinion" and that makes it somewhat hard to progress any sort of debate.
The good news is that opinions can change. This happens all of the time. I hope we can all agree that it is a good thing for all concerned for disputes to be resolved where possible. Unresolved disputes can lead to anything from a broken friendship to global war, so resolving disputes is good, surely.
When there is a dispute it normally means that the parties have differing opinions. Of course, in some cases, the opinions really do not matter, and have no impact on anyone else, so the debate need not actually be resolved and one can "agree to disagree". There are however cases were an unresolved dispute causes further problems for parties concerned, and so resolving it is a good idea. Resolution is, ideally, by rational debate - with both sides explaining the rationale behind their opinion, and considering new factors from the other party, and ultimately one or both parties changing their opinions. Ideally, the outcome being that both have the same opinion in the end, or at least compatible opinions allowing the matter to be settled.
So, where does "everyone is entitled to their opinion" fall down? I think a good start is considering cases where this does clearly apply.
- There are cases where some issue really is totally and purely a matter of opinion because there is simply no right answer. "What colour should I paint the bedroom?" is perhaps a good example.
- There are cases where there may be tests and rules that help establish which answer is better than another, and may help form a consistent opinion, such as "You'll want a light colour in a bedroom". However, even these "tests" or "rules" are often a matter of opinion in themselves, and you end up with a question of what criteria should apply to deciding what is the right answer.
- Some questions have plagued man for so long that philosophers over the millennia have written at length on the ways to consider the right answer. Some times we think we know the answer, but even then there are issues over whether there is a "right answer", such as "Is it wrong to kill a man?" - well, if you are the executioner for a court where there is a death sentence, maybe it is or maybe it is not. What if not killing that man today causes a million people to die tomorrow?
- Some times the criteria for testing the answer are clear cut, well established, agreed by all. It could be that he criteria is framed in the question. In such cases, a right answer, or at least a way to test if one answer is better than another, can be agreed.
- Some times the criteria are clear, but the answer cannot be tested until later. This can be the case on deciding a policy for something. Only later can one say that it was perhaps not the right policy to have chosen.
So, the answer is a "matter of opinion" at the time, but you cannot really say "in my opinion the answer is 1, 1, 1, 1, 1" as that is not a valid set of numbers. It is a wrong opinion if that is the opinion you have, and provably so - just go try and buy a ticket with those numbers! You cannot stand there in the shop and insist "I am entitled to my opinion, and that is my opinion, you can't say I am wrong, give me my ticket". Well, you can say it, but we are back to the "entitled to be insane" comment from the start of my post. You have to agree that you are not really entitled to that opinion, and that an opinion can in fact be "wrong".
Of course, even when you are considering valid sets of numbers, there are "tests" for one set being a better choice than another. All number sets are equally likely, but the lottery organisers have said that some combinations are picked by people more often, like 1, 2, 3, 4, 5, 6, or combinations that make patterns on the card, or values 1-12 and 1-31 as people base them on dates of birth. So picking 1, 2, 3, 4, 5, 6 is a "bad idea" because, if that is the winning draw, it will be shared with loads of people. You should pick the least common sets (which they could not publish as next week they would be the most common sets!).
However, ultimately, there is a "right answer". The actual drawn numbers, when they happen. Ironically, if the draw turns out to be 1, 2, 3, 4, 5, 6, then that was the "right answer" even if shared with thousands of people, as it is the answer that gives the most money.
So opinions can become "wrong", in hindsight, but we are "entitled" to have had that opinion at the time. What gets in to "insane" is stating later, "no, I was right to pick 1, 2, 3, 4, 5, 6" when the draw has now happened and was not that set of numbers and you won nothing. You have to accept that, once the facts are know, your opinion can turn out to have been right or wrong in hindsight.
The problem comes when people try to challenge demonstrable facts with contrary opinions and their only argument is "everyone is entitled to their opinion".
I think we can all agree how we "test" which lottery numbers were drawn and hence how we can test if the opinion on which will be drawn turns out to be "right" or "wrong". But some people do not understand that there are plenty of "tests" that exist for other things. We have centuries of well established scientific method in terms of testing things and establishing things to be fact. We also have well established principles for statistics and probabilities. So even when something cannot be proved, such as proving a negative, we can assess the likely best and right answer.
Some times the answer depends on the question, and there is opinion on what is in fact the right question. Knowing the right question makes the right answer easy to find. "Which came first, the chicken or the egg?" should be an easy one. But you have to make huge assumptions about the question, and the assumptions dictate the answer. Even agreeing that at some point in the past two "not quite chickens" mated and laid an egg that hatched in to an "actual chicken" based on some criteria for defining what is and is not a chicken. But if the question is "any egg", then there were things laying eggs long before chickens existed, so "the egg" came first, it just wasn't a "chicken egg". So you clearly, for the question to pose any conundrum, we mean a "chicken egg"? "Which came first, the chicken or the chicken egg?". Well, depends what you mean by "chicken egg" - how do you define that? If you mean "An egg that contains and hatches in to a chicken", then that first egg came before the chicken in to which it hatched. If you define "chicken egg" as "an egg that was laid by a chicken", then the chicken that laid it came before that egg. The question always has a simple "right answer", but the problem is that there is no "right question", and which question you should ask is a matter of opinion. One perhaps has to go back to considering why the heck anyone is asking such a daft question in the first place!
In a FaceBook debate yesterday the question of whether mediums are fake or not came up. People likened it to religion (which is deliberately untestable) and spouted the "everyone is entitled to their opinion". The problem is, just like lottery numbers, the claims of a medium can be tested. Even something as simple as a locked box that only the dead person knew the combination. There are lots of people that die, and many take secrets with them that can be tested. There are prizes that have been put up and never claimed. So unlike religion, this issue is much more like lottery numbers. But the "test" is massively biased to the mediums - they only need one irrefutable case ever and they prove their claimed skills are real. To assert that they are fake then we have to have no cases of them proving to be real, which is a hell of a "test" to pass. But it does pass - we have never had a case where any of these prizes have been claimed, so mediums are fake. It does not get simpler than that.
We also have people that can do the same "show" as a medium, that are just as convincing, that have practiced cold reading, and are happy to explain how they do it and explain how it is just a trick. It is like any magic trick nobody can explain and so people believe is real - except that one day the trick is exposed and explained and now everyone knows it is just a trick and how it is done. Cold reading is that! It is a trick that you can learn to do, that you can read about, that you can see done and see explained. The trick is exposed and so know you know it is fake and a trick and you can stop believing that it might possible be real magic now.