The BNP have no actual policies on immigration, allowing a debate between the major parties in the presence of Nick Griffin would have allowed him to ride the back of conservative criticsm of Labour policy and witter out his horrible catch phrases.
This was the first time a racist party has been on question time so naturally all the questions are on race. He was brutally exposed on his stance for people who might have seen him as a protest vote, which is what needed to happen.