I think it's incredibly powerful as a tool to see what your player base wants, but the point that sometimes the player base is idiotic is well taken. Just the same, to me what seperates good game developers from great ones is, beyond one or two points about actual game design, the ability to interface with the community, know what the users wants, and at least be able to explain your decisions in relation to that. Like "I was planning on implementing ideas foo, bar, and baz soon, and I did foo and bar first because they are #3 and #5 on the suggestion list, while baz was like #15. By the way, I have no plans to pursue #2 because it would decrease frame rate by an unacceptable amount, and while #1 is a really cool idea, it's not something I'm interested in just at the moment." Even though the communities top two picks weren't implemented, they feel like their ideas got heard and considered, which builds good will. And the developer, who had three ideas, had a tool to decide which idea would be the best received.
Because the development of DF is over-time and donation based, it actually models something similar to an MMORPG- continuous pay for continuous development. For that reason, like an MMO dev, Toady has a vested interest in pursuing the aspects of his project that are most likely to generate donations (so long as they don't compromise his game design, of course).
Personally, I'm really thrilled about Toady putting this page up, and I think it really shows what a quality dev he is.