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.

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