I think what uppercut is saying is this:
For years my parents voted labour but often said they'd like to see the lib dems get power. But the reason they voted labour was to have a realistic attack on the conservatives. They saw the Lib-Dem vote as a "Wasted Vote"
It can also be described in a metaphor from the simpsons:
Well I believe I'd vote for a third party candidate.
Go ahead, throw your vote away!!!!!