As the admin of a Steam group, I would much rather give out Steam credit for group promos and rewards instead of gifting games. Don't get me wrong, gifting games is great for friends and I have done it several times. From an admin standpoint, however, being able to award a group member with some form of credit on account has a lot of nice benefits.
1) The group doesn't have to worry about how members will respond to a particular game title. The credit could be spent on whatever game they want [on Steam]. This reduces the group's risk of members not participating or having a poor reaction because of a dislike for whatever game is being gifted.
2) Unlike giving out prepaid credit cards or issuing the award money though a service like PayPal, Steam credit ensures that the beneficiary puts that money back into the Steam community. It would also be a lot easier to issue since the group already has the member's Steam account on file.
3) It allows better control of the group's budget. It's a lot easier and I assume would be perceived as a lot more 'fair' by members to give away $20 Steam credit every week than it is to hunt down $20 games that members might want (assuming they haven't purchased it already). That way a group can set their promo budget to $80 per month and easily fall within that budget without worrying about the price of individual games.
4) Members could apply the credit to a more expensive game and pay the difference out of their own pocket. A great example is when a new title comes out. Instead of the group having to spend $50 or more to purchase the game as a gift, a member could receive a $30 credit from the group and use that to help offset the cost of the game. This way the group can help members purchase the cool new titles, but the group won't have to buy the games outright.
Basically, a Steam group would be able to manage money and member rewards a lot better with some form of Steam credit. Of course, a lot of these concepts still apply when gifting to friends as well. I think the current game gifting system is great and should definitely stick around, but I think Steam credit should be implemented as another way to gift to Steam members.