In theory you can, but there's a marked difference between filling out literally dozens of descriptions, and simply putting information in a single field. Also, back to RP accounts in particular, in those cases many people when they do keyword icons frequently use descriptions of some kind as the keywords to begin with, since they're largely just many, many different images of the same character. Plus, as someone else mentioned the username is already sitting there right next to the icon, so it's a little redundant...

Basically, while I definitely understand where the issue of what the overall userbase uses versus what the minority uses is coming from, from a purely pragmatic standpoint it makes more sense at least in RP-land to use names instead of usernames in the hover text. Personally, I prefer it that way by far for personal journal use as well, because I've known quite a few people who are prone to changing their usernames every time the whim strikes, and again personal journal icons really aren't likely to feature images that identify the user either. So I don't really see how a good description helps in that case- it identifies the image, sure, but not the user who's using it. And the latter is the information that'd be nice to have without clicking through to profiles all the time.

That's just my 2¢, though. I'm not really expecting it'd make any big difference, but just to give you a little more perspective on the other side of the issue.