@breedmitch, that’s exactly how the convention was explained when the linked @ feature was introduced, and some did stick to that use of it. But others immediately began using it for all references because I guess that made sense to them or they thought they were supposed to. Maybe they just didn’t distinguish between a third-person mention and a second-person direct address.
I was a holdout for a long time for the same reasons you just explained, but after a while I caved because it does make it clear when you’re referring to a username and not just typing some weirdly spelled word. Also it makes them stand out in a thread, in case you’re scanning for someone’s name. So I think this is a case where usage trumps logic and the custom has evolved at variance with the original intent.