Looking through the history, this feature has been around since
MP! 1.40 way back in October 2001.
Trillian had added the feature before April of 2001, so yes they were first, I will admit that, but neither should change a method of doing it after such a length of time.
I concede that MP! would have less users, however 50million is quite significant!
I really believe that MP!L should interpret "/me" as it does its own commands, but I don't think it should send it out differently to what it has previously for compatibility purposes.