Yes there have been posts in the past requesting such a feature. I think it would be good to add that option to HFS. Not everybody will use it, but some people will, so I would suggest that if you impliment it to have it turned OFF by default.
Adding a comment when a file is added to the VFS would still allow the file to be dragged and dropped to other folders and still retain the comment.
However, when a file is added as the result of an upload which is in a real folder I'm not sure if a comment could be added and if it could you wouldn't be able to drag and drop that file or retain the comment to the file when it is manually moved to it's final destination. I don't think adding a comment to uploaded files would give the desired results.