Surely these type of questions encourage debate and bring more traffic to SO.
Because the S[OFU] sites are designed to handle specific, answerable questions. Generally, subjective and/or argumentative (they're not the same thing) questions are not answerable. In fact, by definition, anything which is truly subjective cannot have One True Answer.
Where does this guideline come from? The FAQ for all of the sites:
Avoid asking questions that are subjective, argumentative, or require extended discussion. This is not a discussion board, this is a place for questions that can be answered!
Traffic on these sites comes generally from Google juice -- other people out on the web searching for answers to the questions which have been asked here. Subjective/argumentative questions may potentially boost traffic, but according to what I've heard from those in the know on these sites, they won't boost it by a noticeable margin -- and they do reduce the overall quality of the sites.
(And yes, I recognize the irony of me answering this way, given that I do have participatory answers in subjective questions on at least two of the sister sites!)
The purpose of stackoverflow is to provide accurate, canonical answers to programming related questions (for SU and SF substitute the appropriate topic). While you are correct that argumentative questions might generate a hire rate of traffic, it is not the type of traffic the site is attempting to attract. The site is trying to attract user's who are looking find practical, helpful solutions to their programming problems (and user's to provide those solutions), not users who are "looking for a fight" or to debate what the best programming language ever is (clearly fortran).