I agree with you there. And I pretty much don't trust them, at either end of the scale.
But discussing these things can have some benefits. Sure, you're not going to convert the true believer, at either end. But for someone sitting on the fence, not sure which way to drop, the more information he can get, in either direction, the better his decision will be.
Over the last several years I have noticed that there are many people who have either lost their faith or who never had any to begin with, but were afraid to admit it, either to themselves or others, because they didn't want to be made to feel alone, different. With the more outspoken nature of the atheist/agnostic community, though, these people are realizing that they are not alone, that there are more people just like them. Surely that's a good thing, isn't it?