I'm not sure why it often seems to be GOG vs Steam or Steam vs GOG. I use a variety of stores myself including:
Steam - I've been there since its inception and own more games there than anywhere else. I like the client features and social aspect of what has become more than just a store but a service as well.
GOG - I own a large number of classic games, many for Mac OS X here on GOG. The majority of these were not and in many cases still are not available elsewhere. This place became THE place to get classics for me. It is certainly a plus that all these old and older games are DRM-free and the soundtracks, etc. are nice too.
Mac App Store - I get games here that are not available elsewhere for Macintosh. I'm very happy with the service too which keeps the games updated, downloads and installs them for me, etc. but does not run any client software when I play them.
Origin - Very sparse Mac offerings but good for a few games on sale, The Sims of course and a few Windows games not available elsewhere.
Blizzard - World of Warcraft, StarCraft II, Diablo III & of course expansions to these as they release.
Amazon.com - Stuff like Myth II Soulblighter (used) so I can set it up in source port to play on my Mac. It's a good place to find stuff like this plus I buy all my console games here, some music, etc. I like Amazon a lot.
Humble, Groupees, Indie Royale, etc. - for awesome deals on bundles of course.
Direct from developer - I just got an awesome Mac version of Ultima III for OS X direct from the guy who made it. Every now and then this is the way to go for special titles.
Looking at the GOG sales thread I see people use all of the above and more such as GamersGate and Desura which I don't use myself. It's not that I have anything against them but my needs are already covered by the many logins I have now.
Who shops at just one place really? Even if you prefer GOG or prefer Steam I would think a small minority limit themselves to a single store. I think most people are more interested in the games themselves and the prices for them than which place they come from when it comes right down to forking over the cash.
Nirth: I should know better than to answer a post a like this but just out of curiousity, please provide the main argument that would back this up?
Pheace: There isn't one of course, a true videogame lover would get the game regardless of any possible inconveniences he may endure, since it's all about the game.
Yep. You nailed it. :D