It is something easy to implement but I'd rather not implement it. The list would become quite big with the number of players on and it is a little stalkerish. I don't want people to know when I'm on all the time, I'm sure there are other people who don't want others to know when they are on either.
If you want to know if a particular friend is on, log in and search for their guild card (if you have it). If you don't have a person's guild card, you shouldn't be able to know if they're online or not. Privacy, etc.