Speech that is unpopular today could be widely held tomorrow. Once upon a time, the idea that burning witches was wrong was deeply unpopular. Yet nowadays, thank goodness, just about everyone agrees with it. In the marketplace of ideas, the good, the bad, the true and the false, the popular and the unpopular, should be allowed to fight it out among themselves without being suppressed. This way, so the argument runs, the truth will out.
Even if this doesn't always happen, a moment's reflection will tell us that free speech is generally the best way to go. Just because an opinion is unpopular doesn't necessarily make it untrue, and vice versa. There might be all kinds of reasons why a particular opinion is unpopular, reasons that have little or nothing to do with its truth-value.
That being the case, the best approach is to give each and every opinion, no matter how unpopular, the chance to be examined, scrutinized, and criticized. Under those circumstances, what invariably happens is that we are then able to find out why such opinions are unpopular, and if so, whether they ought to be. Having examined them, we may well continue to reject them, but at least then we'll be doing so from a position of much greater understanding, and for that we require free speech.