I can't work out of the usefulness, except for it being a fun fact. People are probably more interested in the games we play.

It's good when people have dualboot and so you can see on which platform they actually are. Like it says when someone is on a mobile..

There is a usefulness to it, actually.

Imagine, if you will, that you often play Dota2 with a group of people. They see you on and send an invite to their group chat to get ready. You're in the middle of playing some TF2 on Linux and get the message, but you're in the middle of pushing the payload. Now's not a good time to to check the invite, so you let it go unseen. After completing the round, you finally go and message your friend, saying that you're on linux and won't be playing Dota that evening.

That time was wasted. If you had the platform available, your friend could SEE that you're on linux and go from there.

Personally, I'd like to see this implemented in one of two ways. Either show the platform icon next to your name (and have that able to be toggled) or implement a variable-tag sort of thing where instead of having a name be (for example) "[linux]MrSchism], it could be "[$OS]MrSchism" or "[%OS]MrSchism" (showing, of course, the name of the OS). By doing it that way, the user has control of when they want it to show and how others see it.

Both could potentially be enabled so it shows the platform image, plus the changing tag.

It´s not a vital implementation but it would be great in my opinion, the user could enable or disable this feature, it would be very interesting.