I would hope they've thought about that scenario MOnica. In practice, it doesn't matter if you're a LibDem voter, but it's decided that a Labour candidate has a better chance and there's little campaigning for the LibDem.
The LibDem wouldn't have won anyway, so the voter would have been wasting his/her vote. He/she might choose not to vote, which doesn't matter anyway because it was a wasted vote, but those who do grit their teeth and vote Labour could tip the balance and return a non-Tory.
The LibDems have a target list of constituencies anyway and put all their efforts into supporting the candidates they think most likely to win.
In my case, it's unlikely the huge Conservative majority will be overturned. I have never voted for a winning candidate and I have always thought my vote is wasted, so I'm disenfranchised. Neither LibDem nor Labour puts much effort into winning round here, but it would be good if one of them did.