As in the majority of cases the Tories and Labour will occupy first and second place, their other preference votes are unlikely to be counted. As has has been mentioned before, both the Tories and UKIP could be the main beneficiaries. Under FPTP, UKIP has taken a small but significant number of votes from the Tories. Many Tory supporters, though sympathetic to UKIP, refrain to vote for them for fear of a wasted vote and handing the seat to the opposition. Assuming that the vast majority of UKIP voters would vote Tory otherwise, then it would be safe to vote UKIP first and Tory second, knowing that the vote wouldn't be wasted. This can only benefit the Tory Party. I wonder if Miliband and Clegg had this in mind:)