Tories made their bet, times for lib dem to decide.

When avoiding the word coalition, David Cameron made it clear that the conservatives will not bent to any under table deals that give away decisive power on key issues ( like electoral reform). Instead, he simply asking for lib dem non-action to formed a Tory government, with or without Lib Dem.

After failed to materialize the popularity after televised debate, Lib Democratics now facing once in a life time decisive moment, and opportunity.

Trying to form a lab-lib government will seemed like a betrayal to the voting results, If the gov survived and performed well, Lib dem may have a chance to push through the electoral reform that they cried for and hence , gaining a ground to be more important in next election.
But if they failed, lib dem will hit badly in the next election.

Cooroperate or non cooperate with tories will be a Heads I lose tails you win matter.
If lib dem cooperate with Tories and tories governed well, tories will get enough majority next time.
If lib dem dont cooperate with Tories and tories governed well, same goes
If lib dem dont cooperate and tories governing not satisfying, they can put the blame on non-cooperative opposition and cry for bigger majority.
Only possible better outcome for lib dem is when they support tories but tories failed, but that will give labour a good hand.

Lib Dem should try to maximize the chips they have to form a situation that most benefits them, that is to push through electoral reform. On this moment, it is best to wait for Labours and Tories to make a better offer.