(Lighthearted, hopefully)
I love mumsnet namechange feature and one of the reason is this. I personally don't care if I've disagreed with someone on another issue before - if we're agreeing on a different thread and having a nice time, it's fine by me. People are complex and there's usually something to agree/disagree on.
I know not everyone thinks this way and if not for the namechange option, half of mumsnet will probably stop talking to the other half of mumsnet and threads will deliberately go unanswered based on who posted. So how would you feel if you suddenly realised the person you just replied or posted on their thread is that same poster you hate?