Hilarious as it's been to watch the Tory and media reaction to the Lib Dem's sudden revelation that they know the value of the cards they're holding - and it really, really has been - the sad truth is that, like its namesake, a rainbow coalition is more illusion than reality.
It's not enough to scrape across the threshold of parliamentary majority - to actually govern in any meaningful sense you need a majority that can withstand the occasional shock. Such as heart attacks, sex scandals, sex scandals involving heart attacks, expenses fiddling or (entirely principled and in no way motivated by bribes of high office) defections. It's also a bonus if every disgruntled MP in your party (not to mention each of your "progressive" partners) doesn't know for a stone cold fact that there's a blank cheque with their name on it to be cashed at their leisure.
Tory + Lib Dem = working majority. Lab + Lib Dem + Green + PC + Uncle Tom Cobbleigh doesn't. Whether Brown's still around or not isn't the big issue. Whether they can actually govern is. The best possible outcome for a rainbow coalition is that they go more than a year before the wheels come off - at which point they'll have achieved a) an austerity budget that gets everybody upset and angry, b) some major compromises on each main party's core issues, c) nothing else.
There are many political and electoral arguments against a rainbow coalition. But the biggest obstacle is basic arithmetic.