The comments people place under questions almost always seem to be answers instead of comments about the question.
Comments should not be allowed under the question to encourage people to put actual answers in the answer section.
Comments are useful to:
We don't have a lot of tools to communicate with users. Comments are the bare minimum we need. If you want to address the issue of answers as comments (which has been discussed here before) then this is not the right approach.
When Stack Overflow first launched into private beta, there were no comments. You could address other answers using the common @-name syntax.
Things got really noisy. It was hard to follow conversations, and sometimes finding the right answer took a bit of work.
Comments help to break this up into actual answers and meta-conversations.
If an answer emerges in the comments, you can always re-post it as an actual answer - even if you're not the original author (though it's good manners to attribute those who did write it, particularly if you're copying them verbatim).