Politicians already promise what they can't deliver to win the support of enough voters to get past the post. Under AV, they'd have to promise even more to even more groups, to get their second-preference votes, meaning even more broken promises or even more expensive government. Once they were in, they would be under no more pressure to deliver than before. When the LibDems say that AV will make MPs work harder for more people, what they mean is that it will make candidates lie harder to more people. That may not bother the LibDems, but it bothers me.