Secondly, because the rule of law is a critical component of any civilized society. We cannot guarantee to get it right, but we can guarantee that everyone is treated equally. In this election, one party is standing which is not committed to the rule of law. The British National Party would not treat everybody in the same way under the law, and would write laws which discriminated between people on the basis of their race or colour. If you vote, the share of the vote which the BNP gets goes down (assuming you don’t vote for them of course). That, in turn, makes an important statement about how repellent fascism is.
How you vote is up to you. Unlike journalists I don’t have to endorse whoever my employer feels would be most likely to invite my employer to Downing Street. But do vote.