It's fair to say I'm not a fan of the debates - my view is that in the UK we don't elect a government or a Prime Minister but rather elect a representative, so unless my local candidates are going to have a debate then I'm not sure of the relevance. However, the debates are a fact, and probably do have at least some impact on the outcome of the election. And so they must be done right.
The way I see it, there are two possible ways of looking at these debates:
They might be "Prime Ministerial" debates, in which case they really should be a head-to-head between David Cameron and Ed Miliband, because whatever happens in May it's 99% likely that one of those two will become Prime Minister thereafter. (There's always the possibility of injury or illness, or the possibility that coalition agreement requires a new leader - c.f. Gordon Brown. But any thought that the PM won't be from either Labour or the Conservatives is the stuff of fantasy, more's the pity.)
The other possibility is that they're about who might form the next government, which is especially pertinent this time out when a hung parliament looks very likely. In that case, the debates really need to have representatives from Labour and the Tories, plus the Lib Dems, UKIP, and the SNP.
(Alternately, you could argue that the SNP spot should go instead to a representative of the new SNP/Plaid Cymru/Green voting block. Fair enough... but in practice that means the SNP anyway, they being the largest partner.)
Because while the polls don't (and can't) give a definitive indication, what they do seem to say is that any one of those three parties (and, indeed, only those three parties) may hold the balance of power after the election, and so be critical in the formation of the next government. They should all be heard, and in a UK-wide context they should all be heard equally.
Additionally, the fact that the SNP only stand in Scotland is an irrelevancy, for two reasons. The first is simple mathematics: despite standing in only 59 constitutencies, if they take enough of those 59 then they can hold the balance of power. And, frankly, they're more likely to do that than UKIP are to jump from 2 MPs to 30+.
The second reason is this: the people of England (and Wales and NI) may well find their government decided by the SNP, and they therefore need to know what the SNP have to say. Voters in England will most likely have heard the Lib Dem message, and will have heard the UKIP message (thanks to the BBC publicising their every move). And, of course, we all know that the Tories stand for the rich getting richer and that Labour don't stand for anything.
But it's unlikely that more than a tiny fraction of English voters will know anything about the SNP beyond the caricature shown in the Daily Mail and Guardian. Yes, it's true that their main focus is independence for Scotland, but given that five years of government would be about much more than that (and, indeed, in the event of a Labour/SNP government wouldn't be about that), it's kind of important to know more - where do they stand on schools, on health, on defence...?
(If nothing else, if voters in England heard what the SNP had to say and found that they hated it, then they have the power to ensure the SNP get nowhere near controlling the balance of power - just vote for a majority Labour or Tory administration.)
So, that's my take: either the debates should be a Cameron/Miliband head-to-head, or they should involve the five parties who might be involved in making up the next government (Labour, Tory, Lib Dem, UKIP, and SNP).