The Lib Dems poll ratings are roughly the same as the Greens and this should be enough to ensure that Nick's voice is heard even if polls show them (and UKIP) way behind the Greens among young voters. Admittedly Lib Dems are practically toxic to young voters (my 16 year old son does refer to Clegg as "Cameron's Butt Monkey") but that misses one important function of these debates which is to hold the guilty to account. As the guarantors/collaborators with the Conservative government Clegg needs not only to comment on the future (that he won't be a part of) but also answer to us all for propping up a Coalition Government that has cut taxes for millionaires while reducing the incomes of the very poorest who will never benefit from a reduction in income tax. He could also explain how with one of the lowest percentages of energy generated from Renewable sources in Europe he can still claim to be part of he "greenest Government Ever".
I'm sure some enterprising person will set up an online petition to allow Nick to join Dave, Ed, Nigel and Natalie at the debates but I can't be bothered right now as I'm very busy and only catching the news in snatches.