Its a stupid line in a discussion of any issue since it (the ability of anyone to express a view) is almost never the issue being discussed. I discussed the fallacy of this reasoning years ago on this blog but the erroneous thinking still arises and still bugs me.
I notice the issue is discussed over at The Conversation but I think the author Patrick Stokes does not get it quite right. Its not that this line can be used to justify silly or unreasoned views – although that is certainly true – but essentially that the claim is irrelevant to what is being argued. If, for example, the argument concerns the ethical case for legal abortion, and those arguing put a series of reasoned claims but then one party closes with a defiant “everybody is entitled to their own opinion” then this resolves nothing. The argument is not whether people have the right to an opinion but whether, in fact, abortion is ethical. The discussion is won by the person who provides the best reasons for their respective view – the claim that one party has the right to “express themselves” is irrelevant. (1986)