I understand what you mean, but equally it's a quirk of our political system that any party can say what they will do in the manifesto, yet it will only be do-able if they rule in majority government. And as past elections have shown, any party that gets into power can go back on the manifesto after they finally get a chance to see the 'state of play.' Any manifesto from a point of opposition will only ever be at best educated guesses on what could be achieved.
They cannot say what they will do if they need to go into Coalition because they will never be able to certainly predict the proportion of influence they will hold within a government formed in that way. This will devastate the Liberal Democrats in the next election after the farce over raised tuition fees, despite the fact that for the first time in recent history a liberal party has had chance to influence any kind of policy..... just on a minority basis.
It's a problem with the rules for me, not the players.