First of all, I am nothing if not smitten with the idea of using the community to determine the "right" answer amongst many possible answers. It's a tremendous improvement over forums, where there is no way to be right or wrong.
However, given that the central improvement SO makes over forums is that you can separate wheat from chaff, why should SO actively discourage subjective questions, which are often the questions that most need to have people weighing in on them?
Where better to go if I want to get a vote (with substantiated opinions) on the best Rails plugin for a given task? Or the best approach to a problem? Or the best anything? SO's ability to determine the "best" of something is exactly what makes it great for me. Yet including the word "best" in my question gives me that nasty "your question looks subjective and may be closed" warning. I feel like I'm being scolded for leveraging the key strength of this site.
Of course, I suspect I know why SO discourages this. Probably because subjectivity can lead to disagreement, and disagreement leads to time spent managing community. I happen to run a big, community-driven website, and I "get" that keeping people of differing opinions civil can be difficult. But it is a solvable problem (hint: community+technology = automation) that shouldn't be allowed to interfere with one of the key strengths of this site.
At the least, I think it makes sense to re-evaluate trying to scare people off when they put the word "best" in their title. Asking about the "best" programming language is probably a discussion not worth having, but to discourage the asking of any question with the word "best" feels like throwing the baby out with the bathwater.