Both steam and direct2drive work very much the same way in that they are both stores. Online download stores. The difference is Steam (ran by Valve) uses management software to allow everything you purchase to remain on your steam account that can only be accessed through the Steam program. Direct2drive (ran by IGN) stores your account on a secure site, but downloads require the fileplanet download manager. The advantage of purchasing games off of any of these services is unlimited downloads for backup (the game can be stored in installer format on your pc or kept on the server) and the game no longer requires a disk. Another advantage for developers is that the software authenticates online through your account so that no-one else under another account can use your serial number thus you can't distribute the software unless you truly want to be caught.
The problem many tend to think is that Activision may not be willing to expend resources needed to modify Redemption so it can be distributed through either of these online download stores. I'm still looking into this possibility, but if Activision does pull it off then I'd be willing to continue mod for the Redemption game. If Activision decides not to actually sell Redemption then I'll pack my bags and move to HL2. Actually, I'm already moving to HL2 so I guess I should say, I'll return to Redemption when Activision decides to sell it again.
I see no point in modding a game that isn't even readily available to the public anymore.