Well, in one sense, yes of course they do. By putting Vince Cable and, later, Nick Clegg up against their Conservative and Labour peers the Lib Dems are granted a status and respect they never achieve in other circumstances. So in terms of exposure and credibility then yes the debates help the Liberal Democrats.
The format helps too: since Labour and the Tories will sensibly ignore the Liberals the third party is rarely tasked with the awkward business of defending its own proposals. Instead it can scamper around picking off the low-hanging fruit dangling from the Labour and Toriy trees. Since, god help us, there's no shortage of that then even mildly competent Liberals ought to be able to enjoy themselves during these affairs. In a three-sided "debate" there's nothing easier than being the "plague on both their houses" candidate*.
Conventional wisdom has decided, I think, that all this means the Lib Dems will benefit most** from the debates. Well, perhaps. But while Cable did well on Monday and while more people will recognise Clegg after his debates with Brown and Cameron I'm not sure that much of this will have any real impact upon Lib Dem support. An impact on Lib Dem approval? Possibly. But not on actual votes.
At best, I wonder if the increased publicity from the debates will only help the Lib Dems hold onto a handful of seats they might otherwise have lost. The reason? People have already put a price on the value of a vote for the Liberal Democrats and in most constituencies have concluded that it's not worth very much. Consequently, I suspect plenty of voters will ignore Clegg et al and discount their debate performances. It doesn't much matter how good Clegg is since he's not going to win the election.
The Lib Dems suffer from a collective action problem. If you could be sure that lots of other people would vote Liberal Democrat you might be much more inclined to vote Lib Dem yourself. But you can't be sure of that and, in any case, all these other potential Lib Dems are wondering the same thing and concluding, perhaps with a mournful little sigh, that they'll not vote for the Lib Dems either.
Perhaps super performances in the debates will change that but I rather doubt it. It might help the Lib Dems in a handful of seats but as more and more voters take a national, not local, view of the election this won't do much to boost the Cleggers unless, that is, all of a sudden several million people decide Nick Clegg could be Prime Minister. I must say that I think this improbable.
In other words, "Do Debates Really Help the Lib Dems?" is, I suspect, a Question to Which the Answer is No.
*Bloggers, mind, take a more ecumenical view still: a plague on all their houses.
**I believe I have made this point myself. I may now have changed my mind.