I suppose that the whole thing might at least expose once and for all that they are all exactly the same. But you can't stretch that out for nine hours.
Of course, if we must have these things, then no party contesting fewer than fifty per cent plus one of the seats should be on them. What on earth for? But by all means let there be further ones in Scotland and Wales featuring the SNP and Plaid Cymru, though not with whoever purports to be the Labour, Conservative or Lib Dem Leader there, but with Gordon Brown, David Cameron and Nick Clegg. And let the whole country see, as no doubt we would on BBC Parliament if nowhere else, the look on the face of Alex Salmond in particular as Brown nonchalantly mentioned "At the G8", or "While I was chairing the UN Security Council", or whatever.
The problem with having the BNP on would be that no one would talk about anything except the BNP. As for UKIP, its response indicates once and for all that it is now just another Tory pressure group. If Cameron or his successor (but no one else) promised an In/Out referendum (but nothing more, or any issue), then UKIP would disband. UKIP is a non-domiciled party for people who may not be members of, or voters for, the Conservative Party, but for whom it will always be their natural home. And as of today, once and for all, it regards nothing but the EU as worth debating, not even in the run-up to a General Election.