Mike Gene has come up with the 10 signs of intellectual honesty, which is a very nice thing to strive to be. Unfortunately, most of us over rate how intellectually honest we really are. And even if we were completely intellectually honest, we are still fallible human beings with a finite amount of knowledge.
So how do we know that our beliefs are true or at least reasonable? One of the ways to test them is have people who oppose our beliefs attack them with every argument they can think of. It would be nice if these people had a cheery attitude about it: "I'm sorry, old chap, but it looks like there are a few problems with that belief of yours. Don't take it personally. I have nothing but the highest opinion of you, but…." That would be the ideal attitude of a critic. And it would also be nice if the critics were completely intellectually honest themselves, and of course, if they were experts in the relevant subject.
But the people who are most likely to attack our beliefs with every argument that they can think of are the ones who most oppose our beliefs. And rarely will they be very kind about it (how difficult it is to be kind to some who holds such idiotic views), and not always completely honest. But the one thing that probably matters most is that they are experts or at least have a good deal of knowledge about the relevant subject (especially here at TT, where most of us are amateurs).
We should value these people more than gold and do everything we can to keep them around. It isn't easy. They will often say things we don't want to hear and in ways that we find offensive. But if want to test our beliefs, they are really our greatest friends.
This is why banning critics at Telic Thoughts should be rarely if ever done. It hurts us much more than it hurts our supposed "enemies."