There are two ways in which you can view a Question/Answer site. One is as a real-time "I've asked this question and I want a quick answer now" type of site, and the other is as a longer term repository of questions and definitive answers. I think that many of us here take this latter view of the site. As a result, we have lots of very high quality answers, and a relatively low number of competing answers for many questions. We also tend to comment on answers to improve them rather than posting competing answers. This is a very good thing, IMO and not only makes the site a useful long term resource, but also fosters a real sense of community where people are willing to help each other rather than fighting over reputation.
I think that having lots of competing answers to any particular question just generates noise and makes the site less useful. Also, as you can see from other sites where this practice is more common, when there are lots of answers, early answers tend to get upvotes and very good but later answers sometimes don't.
The commenting facility however, provides a real time remedy to this situation: a comment that also provides an answer may solve the original questioner's problem, and (as others have noted) at the same time provide time for us to find and link to other related or duplicate questions. This again adds to the value of the site.