the one thing that AV will do is allow far more protesting voting. you can happily vote for Green/Socialist Worker/UKIP/Raving Looney/BNP/independent, happy in the knowledge that only your second vote will actually count. if we see upswings to say 5, 7, 10% for these fringe parties nationally, maybe it will press the case for PR.
This is very much my opinion, it only benefits the protest vote.