I hate to say this but Steamworks is becoming a requirement for game development these days. I'm hoping Gamestop is able to get Impulse::Reactor finished soon but it's really expensive to do in game data tracking and such.
Personally I don't think it will make a huge difference for GS, but I think that would certainly be great for publishers. The only way gamestop (at least in my mind) makes good on impulse is by using their stores to promote impulse. For instance, trade in a game, get $70 credit at impulse vs $30 in store (at least for me, it would need to be that good). If they don't come up with some strategy (and they should...), then it obviously wasn't the best purchase. That said, gamestop's been pretty freaking successful... of course, so was blockbuster. Executives did a great job on that one - who cares about netflix and redbox - we'll just open more stores...
Honestly, I'd only buy something from impulse if I had an incentive like I mentioned above or they somehow proved to me that they offer better deals than steam more often. As I very much doubt they will put the money into marketing and running excellent sales as often as steam, that really leaves using their huge B&M advantage to push impulse. But they have to commit to taking a loss to grow impulse at the store. Will they be willing to do that? I'd say yes (if I was them), but I really don't think they will.