Of course they should be allowed on but I hope they are given a proper opportunity to answer questions. There was a Jeremy Kyle episode I caught once where they had a paedophile on (blacked out.) He kept trying to talk but they just kept shouting him down and calling him disgusting. It made me think that there was little point in having him on in the first place if they didn't let him speak.
I really hope they don't have person after person calling the BNP disgusting to raucous applause. Ask them sensible intelligent questions and let them make a rod for their own backs.