With reference to pdq1's comments, UKIP would be of no use to the Tories (or anyone else) in a hung parliament as they'd have no seats.
UKIP are basically Tories who put Europe at the top of their agenda, and they stand to deprive that party of potentially vital votes in marginals. Personally I think the threat is exaggerated, but on the other hand if a potential UKIP voter has no UKIP candidate to vote for, who he he/she going to vote for instead? Most likely a Tory, or at any rate, the most euro-sceptic candidate.