When I log onto the server looking for a game, it is nice to have the feature that tells you who is online with the same plugin, however I have been online and was willing to play any game that I have as a plugin. I think a friends feature would be nice for this.
If I log on and see that someone is playing a game I like but they are tied up. I could save his name as a friend and quickly see whether he is on again later. And if they are not maybe I could leave a message with him asking if he wants to play a game later.
This will not make or break the program, but it would certainly bring me to the server more often as it is hard to find someone on with the same plugin as me at the same time, open for a match right then and there.