Speaking personally, I voted lib dem in 2010 on the basis that they were a party broadly of the left, but distinct from the Labour party of the time in a number of positive ways. To then form a coalition with the party of the right and support a number of measures that I think were quite regressive did not match with the party I thought I voted for.

Of course after seeing 10 days of government without the lib dems to moderate I'm starting to re-evaluate how positive an influence they likely were...