Having now looked into the figures, I think the "horse-trading" argument against AV is a red herring. I think AV will deliver majority governments at least as often as FPTP. There's an established process under FPTP for forming coalition governments (who can deliver majority government which is therefore able to pass legislation) which I don't imagine would change much under AV. So the horse-trading will continue (as it does within factions within parties anyway), and presumably happen as infrequently as it does under FPTP (every thirty years or so).

ETA: All of which leads me back to my original position: why bother?