You will need one to mine Valve's hatcoins
It seems like they would have something that Bitcoin doesn't, a lively economy and marketplace to add stability.
You'd be better off introducing a Call of Duty Coin... Call them Dogtags or something.
You can check Steam user counts here (http://store.steampowered.com/stats/).
Not sure how console user bases stack up, but that's a pretty wide base.
Wow had 12M at its peak.
And there is huge overlap. LoL gamers and WoW are subset of steam users.
http://steamcommunity.com/market/
Most hats aren't saleable on the market, but you can trade the hats for keys, which you can sell for about $2.25 a pop.