It didn't really follow that the debates helped Clegg. At least, not at the polls, when the predicted "surge" didn't really materialise and the voting share for the Lib Dems barely increased -- indeed, they ended up losing seats. The debates this time round might probably have the same, or a similar, sort of effect, namely a quick popularity boost for Farage (and perhaps even the Green Party), that doesn't go all that far at the polls as, overall, voters are likely to have made their minds up anyway.
In the current situation no-one other than Cameron and Miliband has a hope of being PM anyway, so really the debates should consist only of those two, if they happen at all. I actually think that such debates should happen but only as part of a larger change in the way our democracy works.At the moment, our democracy is sitting rather closer to presidential-style politics while at the same time pretending that we're voting for our constituency MP. Really, we ought to make our minds up whether we want to vote for a PM and his party, or for a local MP, and either do away with or strengthen constituency-based politics accordingly.
On a technical point, US voters don't vote for a president either, and the system actually has them voting for the people they want to vote for the president. It's just that in most cases the people who choose the president go the way they're told to go, so that everyone tends to forget the middle men (electoral college) in US elections.