The voting system has given me valuable feedback, and I belive changing system in the way proposed would make it worse, overall.

When I express an idea to a group of people in person, I may get some comments, but I also get some nods, smiles and non-commital expressions of intrerest. The PM voting system is the first thing I've found that gives me this feedback on-line. It's valuable information, even if I don't know how closely the people voting were paying attention. Even in person, I don't know if someone's nodding in response just to cover the fact that he's thinking about something else entirely. Mostly they are paying attention, and mostly voting on PM seems to work.

The average quality of posts here is impressively high, and I wouldn't like to see that diluted by an incentive to post something. I also wouldn't want to lose the vote feedback, pretty much as it is. Yes, XP ambitions can provide an incentive to vote carelessly, but it's a pretty mild incentive. If it were any less strong, it wouldn't be much of an incentive for newer users to vote responsibly.