It should be quite clear that in this particular instance, Valve has decided that Bitcoin has utility as a transfer of value. You're questioning whether or not Bitcoin has utility as a store of value / unit of account. It appears that to Valve it does not, but there is also a chicken/egg issue with bootstrapping a new currency given that Bitcoin doesn't have sufficient adoption yet to complete a full lifecycle in terms of usage. That is - Valve has no use in holding bitcoins because they can't use them to pay their bills. This may change at some point.
At this point there are many people who simply have bitcoin. Maybe they bought it a long time ago, maybe they bought it recently. The acquisition time isn't really a factor anymore, it's been around long enough that people simply have it, and they would like to be able to spend it directly.
There are not many people specifically acquiring bitcoin just to pay for steam games with it, where it will be converted right back. Maybe some to test. Questioning the value as a result of the "usd -> btc -> usd" transfer isn't really applicable because it just generally doesn't apply anymore.
