Does Valve really needs more than 0 confirmations? double-spending doesn't seems like a big issue for them since Steam is DRM and they have some level of control over the software people have in their libraries (and total control in the case of online games). If the concern is whether some people would just download games, double-spend before getting one confirmation and then keep Steam offline forever (or copy the game files from the steam folder), I think people can already do something similar as they let users "return" games during the firsts 24 hours after the purchase.
The problem with zero conf now is that the transaction may never get through. Blocks are full so often that if the fee isn't high enough the tx might stay stuck in the mempool for weeks until the machine is rebooted and the tx is lost forever.
Extremely low tx fees may never get into a block, but I just checked the the current price for a target of 24hs max (the same window time that fiat users currently have for returns) and it shouldn't be more than $0.35 USD for a simple 1 to 1 transaction (14 sat/byte). I know that this still isn't good, specially when you may be sending just $5 for some cheap game, but it's not that terrible either in my opinion...
Problem is that the minimum fee for 24hr target could change a lot in an hour or less because some news story happened or simply when enough people noticed a lull in fees and decided to move their fees when it's still cheap. In that event 24hrs coild easily turn to days.
Wallet developers and payment portals gets a lot of flak for overcharging fees, but ultimately it is still preferrable to using a conservative estimate and deal with the likely support call when the payment fails to confirm, and there is nothing they can do but watch it get bounced between nodes.
