Themed blogs are not conducive to rational discussion. Exactly the opposite is the case: they seem to allow people free rein to their prejudices. Over and over I see a voice that slightly deviates from the line taken in a blog construed as "trollish". I myself have just been accused of "wanting to start a fight" because I sounded a note of mild context-giving to a particular debate on a BBC-bias blog.
It was the same behaviour I've seen before, and it is anti-debate, anti-rational. People will simply become yet more entrenched in their lopsided views as a result. There's no doubt a blog or site for every misguided world-view around, so you can agree with like-minded people instead of having your views challenged and in the realest sense possible 'rationalised'