it's a fundamental conceit of any democracy that it represents the people. Our democracy does not. The First Past the Post system favours parties with a loyal minority and means that the majority of votes tend to be wasted. A party can win a seat with as little as 30% of the vote - or less - provided it has most of the votes in any given seat. This means we can have seats represented by MPs that most people didn't want or wouldn't even tolerate and if you carry this upward we get issues where the government in power isn't represented by the majority of the people. Incidentally, this is also what happened in the US due to the electoral college system when Bush was elected over Gore (who had the popular vote) and we all know how well that turned out, right kids?
Now, way back in the mists of time when dinosaurs ruled the Earth and party was less of a big deal the local link between your MP and his constituency moderated this problem somewhat but with parties taking pre-eminence and party whips becoming more important the idea of a local link is really little more than a farce when it comes to national policy. Your MP will virtually always follow the party line rather than voting with his conscience or the wishes of his seat.
AV (alternative vote or instant run-off voting) is a 'miserable little compromise' between FPTP and true proportional representation, but it IS a step forward. Now your votes won't be wasted and so long as you express a preference (which you don't even _have_ to do) your vote counts. Everyone's vote counts and we retain the unicorn (sorry, not unicorn, the link between MPs and constituencies).
So, why would anyone vote against this or not vote on this?
1. Votes don't matter!: You're a bellend. This is a referendum, EVERY vote matters on this vote and it means votes will matter more in later elections. If you believe votes don't matter this is a way of beginning to address this.
2. FPTP favours my party!: You're being a bellend. This is about the nature of democracy and its representativeness, not what suits your mob. Do you believe democracy is a good thing or not? If we willingly allow it to be distorted because it suits us, that means we're not really any better than non-democratic nations or sham democratic nations like Zimbabwe.
3. I want PR, I'm not voting for AV!: You're a bellend. By abstaining or voting against AV your stance is going to be taken as tacit support for the status quo and you know it. This is our once-in-a-lifetime chance to get some impetus going on reform, not just for voting and moving towards truly representative systems but also reform of the Lords, helping reduce corruption, making less seats 'safe' and a whole lot of other stuff. If you want reform of ANY kind you should back a move to AV.
4. It'll squeeze out the smaller parties: You're being a bellend. We can't know quite what's going to happen but since people can vote HOW THEY WANT and not waste their vote we're likely to see something of a shakeup of the political landscape. People will be able to put smaller parties as their primary vote without neutering their say. That'd probably mean more Greens and Libs (once they recover their reputation) and whether you like that or not, it IS more representative which is the point of democracy, yeah?
5. OH NOEZ COALITIONS!: You're being a bellend. We get those under FPTP as well and while the Libs have failed to be The Conservatives Jimminy Cricket effectively, coalitions do tend to moderate the ideological excesses of their participants. An end to adversarial politics isn't necessarily a bad thing. Making these goons work together is a good thing.
Don't be a bellend, vote yes to AV.