A lot of comments seem to be perfectly good answers, so why not create an answer? Is it to be the first read response? Is it that you don't think your opinion is good enough to be judged as an answer? Do you not seek the sweet sweet thrill of watching your reputation points go up in 10 point increments?
And yes before you ask, these things do keep me up at night, disturbing my circadian rhythms.
When I'm in a hurry or in the mood to write a response worthy of a twitter tweet then I leave a comment. When I want to be praised for my tremendous insight and show off my reputation score I write an answer.
Answering with a comment to the main question can help ensure your amazing and cunningly articulated opinion doesn't disappear too fast. Comments to the question are probably read first too. I know that I often won't bother scrolling all the way down to see all the late answers--especially if some of the early answers received lots of upvotes.
Meantime, you can't get downvoted for comments. Since we've seen plenty of downvoting for offenses no greater than having an opinion that differs--no matter how respectfully it was offered--I can see why some might want to comment.
Now, kindly upvote my answer, since it is clearly the most definitive, comprehensive one, and I'd really like to hit 3,800 points by the end of the day. [Edit: I upvoted this question. I'm just sayin'....]