A number of arguments keep cropping up from left-wing supporters of AV. Here's a brief, last-minute response.
1. Is AV, while flawed, still an improvement on First Past the Post?
No. AV is not a form of proportional representation. The constituency system remains entirely intact. AV is no fairer than FPTP and - crucially for the left - it doesn't make it more likely that left-of-Labour candidates will be elected.
2. Is AV at least a welcome move towards proper PR?
There's no reason to believe it is. Winning mild reforms can be a spur to further action, but it can also be a way of saying "This far - and no further". The introduction of AV would more likely be used to block any more far-reaching change. The Labour and Lib Dem leaderships alike are not keen on pushing any further than AV.
3. Does voting no to AV mean allying with the Tories?
Obviously not. You could just as easily pose it the other way around: the Tories are allying with sections of the left by rejecting AV! But, of course, there are radically different motivations for opposing AV. For the Tories it is a combination of anti-democratic sentiment (which means an implacable defence of FPTP) and electoral self-interest. For the left, opposition stems from the way AV props up the centre ground of politics, offers a lifeline to the discredited junior partners in a viciously right-wing government, and does nothing to enhance democracy.
4. Should we vote YES in order to "punish the Tories"?
David Cameron won't be too troubled by the passing of AV. It will do no damage to the health of the coalition. A defeat for AV, however, will be devastating for the Lib Dems, especially when combined with the poor results predicted for the party in the council elections. AV is a shoddy deal designed to help the Lib Dems. It was central to Nick Clegg managing to carry his MPs, councillors and grassroots activists with him into coalition with the Tories. Defeating AV will sharpen tensions and deepen divisions in the coalition government.
5. Is this issue more important than 'short-term considerations' like wanting to damage Nick Clegg and hurt the coalition?
No. It's not the UN Declaration of Human Rights. This is a dismal proposal we're only voting on because of dodgy dealing between Tories and power-hungry Lib Dems last May. It isn't as important as the future of this coalition government and its ability to impose savage cuts on us.