When I post a comment to a reply, the 15 minute timer disappears when I reload the page. I goof occasionally with spelling, and I'll admit it, in my answers sometimes, so not being able to fix something minor (or major!) is a bit harder than it used to be.
Also, comments on replies don't send email notifications, which means as a Support person, it'd be easy for me to miss a reply to something if it was posted in comment form.