Well, the transaction fees will be too high for micro-transactions like skins, weapons, virtual currencies, etc... Especially for mobile games. Plus, the number of raw transactions will be way too high, there's like 2 billion smartphones nowadays. Do you really want to flood the bitcoin blockchain like this ?
Using alternative currencies for online would be a better solution. Plus, you could add really specific features. Just look at Enjin, it basically gives a way to trade your in-game items in a peer-to-peer fashion, without having to rely on the official servers. Like, if a pokemon game (pokemon go, for example) uses EnjinCoin to power up its transaction system, it would be possible to trade pokemons like a real pokemon trader, like, your wallet will have a pokedex-like interface and, yeah, within seconds you can send your pokemon on your friend's cellphone. Like, ehm, pokemon go, but with, ehm, better playability and social mechanics.
Enjin is the future of online gaming.
it creates a side-chain for any item, and makes generic conversion, so that you can trade it with any other side-chain token. Ka-boom. No traffic congestion. Endless retro-compatibility. Forks possible for updates, or for server tests, you can easily do anything. And you can't get stolen by random chineses farmers who got access to your account, because now you also need the wallet private key to do so.
The Ethereum team also wants to create a mobile cryptocurrency. They did it on a completely new blockchain because there's too much potential users for such a cryptocurrency. Billions of users. We need another blockchain for that.
AltCoins can be useful this way, but you don't need to make small transactions of BTC if you want to use the main Crypto-Currency. If the games start accepting BTC to buy in-game items you would have your balance on the site, so you could send a higher amount of satoshis to make the transaction worth. Not necessary to spend all the money at once. Also, if it's hard to see sites accepting BTC, now imagine how hard would be to see these sites accepting AltCoins...