On paper a Rainbow Coalition of almost every party that isn't Conservative may seem like a great one. It means that you have a wider scope for consensus politics to come to the fore and for partisan politics to be reserved for the would-be Tory opposition. Furthermore, the idea of the Greens and the SDLP being in government would be refreshing in its own right.
The biggest argument in favour of the Rainbow Coalition seems to be that if you add together the Labour, Liberal, Green, SDLP, Alliance and Independents (perhaps the Plaid Cymru and SNP votes too) that it commands the higher proportion of the public's mandate, estimated at 55% by a colleague, compared to the Tories 36% share of the vote.
The problem with this argument is that 55% of the voting public did not vote for a coalition of all the parties that aren't Tories - they voted 29% for Labour, 23% for the Lib Dems and 1% for Greens.
It is worth noting that it is indeed very pragmatic of the Labour Party to try and get themselves into government by any means, I would probably try and do the same myself in their shoes, but don't try to con the public into thinking that it is fair, just and what the public actually asked for.
As a final comment, one a Burnley Fan maybe could appreciate, imagine that, in one final attempt to avoid relegation that Burnley and Portsmouth joined into a team - Burnmouth. They would have ended the year on 49 points condemning, West Ham and Wigan to relegation in the process, in spite of losing 48 games between them!