I dunno, what is the underline really useful for? We already have things that emphasize words, like italics and bold. It is good for referencing books, but italics works the same way.
I can see why you would suggest this, but there really is no point. Bold would work just as well, and it would be too much work for a litle thing like this.
Underline is pretty pointless... but can be useful for highlighting things. Maybe we should have the option for it but only on our profile pages because I know I would use it.