That's a good argument to excuse the membership requirements but the big one in my opinion would be the provision of services to constituents.
Surely you can't be someones elected representative and refuse to represent them? I saw Nick Griffin dancing around that issue an interview following the EU elections. His argument was that due to PR there would be multiple MEPs they could contact, so they should see one of the other MEPs that arn't busy tracking down all the Muslim peodophiles (slightly paraphrasing him but I'm assuming he'd also leave all the no Islamic paedos for them to deal with too). That wouldn't hold up if we ever ended up with a BNP MP so in my opinion that would be a reasonable reason to refuse to let them take their seat.