I've had (and still have) a lot of users coming to me asking how they could comment on an issue. Our typical use case involves lots of comments on an issue, rather than frequent attribute updates, and the commentators are most of the time not familiar with software project management software. It's also odd that you have 2 distinct sets of rights for commenting on and updating issues, but still only one button for it.

Anyway, I think the point here is that said people wouldn't click on "update" because they told me they didn't want to update the ticket, just to comment on it. I think a good compromise here would be to add a "comment" link, either in the contextual menu in the upper right content pain, or to stay consistent with the "forum" and "news" panes at the very end of the content. You wouldn't even need an extra form for the comments, just make the "normal" update form appear when clicking on "update", and keep the "attributes" part of the form hidden when clicking on "comment", with the option to make it appear as needed.