I don't think you quite understand...
1) The lib dem MPs have a mandate from their constituents to try and impliment as many policies as they can, by forming a coalition they can do this.
2) The lib dems want PR. This means that they must show that a coalition can work and that they can compromise! If there's one thing I've learned in this election is that a LOT of lib dem members don't seem to understand that with PR compromise must happen on both sides. However, I think people are starting to get it.
3) We do need a stable government who is able to do things, a minority tory government would not have worked as well, as the LDs/Lab could have toppled it easily and a "coalition of the losers" would not have pleased the electorate much. At the end of the day,the lib dems had to make a choice and in my mind they made the correct one (democratically speaking), they aren't really wangling their way into power, if there had been a confidence and supply agreement the Lib Dems would basically just be backing tory policies, with this situation, the Lib Dems are actually getting some of their policies implimented!!!