Precisely, Wolfgang - we know how to judge what weight to give to someone's opinions on one subject by the quality of their contributions on other topics. Otherwise it's just graffiti.

It's not a question of politics such, but of whether someone's arguments are coherent and thoughtful. The fact that I might agree with someone's politcal stance doesn't in itself mean that I necessarily have much regard for their judgement on other matters - and vice-versa.