PR generally leads to coalition government, and coalition government is generally less stable than single-party government. Stability is good for economies. Furthermore, an argument often used to support PR is that the seats to vote-share ratio is usually closer, and that as a result your vote 'counts more'. However, if it leads to coalition government most of the time, then that means manifesto compromise between parties, so that the policies which lead you to vote for a given party may end up being dropped altogether, which leaves many people feeling robbed. For example, there are a lot of people who voted Tory on the basis of their taxation plans, and who now find those plans essentially scrapped as a result of the coalition deal. There is more than one way devalue someone's vote.
Bear in mind that I live in Ireland, where we use PR and nonetheless have generally had quite stable government. I'm certainly not wholly against PR, as in some respects it has significant advantages over the current UK electoral system. However, the fact is that looking at traditional and current voting trends in Britain, if PR were introduced there would be coalition governments for the forseeable future, bringing with them the above problems. The current system tends to deliver majority government, and that has advantages of its own. Neither system is 'better' than the other, they simply have different pros and cons.