I think the main reason this is being done hasn't been explored yet. It opens the door to easy micro-transactions in free to play games. Gabe has gone on record stating that he wants Steam to do more in that market, and currently it does nothing at all. A wallet would make it much easier (and probably be required) to engage in micro-transactions via Steam, and they could integrate it via steamworks into any F2P game that wants to use its service. Micro-transactions are actually pretty taxing on the vendor due to all the overhead if they don't have such a system in place. This is the first step to Steam embracing Free to Play games.