Because AV is fairer, more democratic, and ends tactical voting.
It’s fairer because never again will the most unpopular candidate win, as they do in council elections all the time, and even in a few general elections (take a look at Scottish elections from 1970, where the anti-Tory vote still eclipsed the Tory vote). If there is truly a “progressive majority”, as last year’s elections apparently stated, then representation in the Commons will reflect that. The same applies for a “conservative majority.”
It’s more democratic because it gives the voter a bigger choice. Instead of being forced to pick the candidate with the most chance of winning and more agreeable with you, you can say that you want one candidate, but wouldn’t object to any other candidate you preference. This is the beauty of preferential voting: it’s more democratic than the simple “X” against a candidate. It’s no surprise that Labour campaigners for the No vote are the most tribalist Labour MPs and councillors. They want the central party to control the entire country, not your votes.
And it ends tactical voting because you don’t have to go with that candidate who could beat the Tories/Labour. I was a tactical voter last year; I could’ve chosen to vote at home, in the Calder Valley, or at Uni, in Leeds North West. I voted for Greg Mulholland – a brilliant MP who I’ve met personally – because the Lib Dems were way out in third in Calder Valley. I wish I could’ve helped the Lib Dems in the Calder Valley, potentially making it a Con-Lib marginal next time, but I had the feeling by vote would have been wasted.
I’m not going to lie, it looks like a disaster; YouGov have us twenty points down, no thanks to attacks on Nick Clegg’s character (despite a gentlemen’s agreement by Cameron not to) and a terrible campaign by the Yes side being saved too little, too late. But if you believe in reform, in democracy, in never seeing Conservative or Labour hegemony on 36% of the vote, vote Yes. It’ll be our only chance for a generation to do so.