Yes, this has been possible for a long time, and I could never decide about it.
On the one hand, it didn't seem appropriate to remove information just because an answer is converted to a comment. On the other hand it doesn't seem right to let people create formatted comments through the back door.
Anyway, if this is an issue on your site, the best solution is to create a plugin with an event module which listens out for the 'a_to_c' event. You can check if ($params['format']!='') in your module's process_event(...) function - if so, qa_post_content_to_text(...) and qa_post_set_content(...) in qa-app-posts.php should provide the functionality you need to perform the conversion.
If you happen to make this, I think other people might like it too!