The Green Party are an established party for many years with an MP who was voted Politician of the Year in 2007 - why are they not invited?
This is not a democracy - its bollocks!
The one thing we have to get perfectly clear in all of this is that it's the host broadcasters (and given there are supposed to be two debates I assume thats the BBC and Sky) who set the rules and invite the participants.
Now for the reasons I listed above Cameron really doesn't want these debates (or more accurately he doesn't want any debate other than one between himself and Milliband because he figures he will win that one). I assume he knows that the broadcasters don't want too many people in these debates because it will slow the process down but they (the broadcasters) have to invite the three main parties and it would be ridiculous not to include UKIP as they are the heir apparent to the Lib Dems 3rd place. So by running with this insistance that the Greens are involved, which the broadcasters won't like he figures he can get the mass debate cancelled and just end up in either one or more debates 1-2-1 with Milliband.
Now on the flip side of that coin Milliband knows he will struggle 1-2-1 (because he does every Wednesday lunchtime). So he needs the mass debate to mortally wound Cameron. Yes labour will be hit by UKIP in certain parts of the country but nothing to the extent that the tories will. What He will do if the broadcasters refuse to invite the Greens could be interesting though.