I'm thinking of voting yes for four main reasons:
1 We've tried FPTP for years and it's clearly unfair - it leaves thousands of voters in safe seats totally without representation.
2 I think AV would have the effect of diluting the power of the two major parties. I imagine the reason the Tories and many Labour politicians want it is because as well as being slightly more likely to perpetuate a two-party system, it is also much more likely to concentrate power in a very few hands once the elections are over.
3 I think it's time for the smaller parties to be given a voice - as they also represent a lot of people's views. Even if AV doesn't change the actual results of the election, under AV the smaller parties will have far more power to influence the policies of the government once it has been elected, than under FPTP. This would benefit the Greens for example, but not the BNP because no party could possibly allow itself to be seen to be influenced by racist views. On the other hand, all the parties want to be seen to have greener policies.
4 Finally, I have completely lost trust in election promises. I think the parties just say what they think will get them elected and then bring in policies for which they have no mandate. Under AV, they will be held to account more, and will have to honour their promises.