Don't bitcoin transactions take a while to verify?
I think the transaction time that you are talking about is the time it would take to convert dollars or euros into BTC. The actual transaction time is fairly marginal.
Also people stop playing games all the time so tons of money would just get lost in the system, maybe if when you logged off all the money got sent to a central wallet, then it could be redeemed when you logged back on? The game's revenue could come from banned account's cash or user's who haven't logged on in a year etc.
Good point on the people who simply would not ever use the coins. Probably would want to bank them at the game's website, then allow people a cashout process if they choose.