Most of us have MPs most of us didn't vote for. Because First Past the Post enables candidates to win with the votes of 1-in-3 people in a constituency, extremist parties such as the British National Party have more chance of being elected under FPTP despite most people in an area opposing them. They've snuck into town halls across Britain and they'd like to repeat the trick at Westminster.
With AV, no-one can get elected unless most people in an area back them. The risk of extremist parties being elected by the back door is eliminated; that's why the BNP are campaigning vigourously for a "No" vote in May.