I made this thread two years ago, http://www.giantbomb.com/forums/editing-tools/820/should-download-services-be-added-as-platforms/10504/
and looking back at it now, I was definitely wrong about the different PC online distributors, as those really just sell PC games. It would be like adding amazon as a subplatform, not sure what I was thinking.
but Steam has since become completely autonomous as a PC platform. It has its own DRM with DLC and an achivements systems seemingly completely compatible across from Xbox Live stuff. If you buy a Steam game you're completely locked into Steam, and are going to have to boot up that application and log into your subscription to play the game. Retail purchased box or not. Although that was also true two years ago, Steam has gained a significant market percentage for the Steamworks DRM and retail exclusives since then and a lot of big publishers and developers are going with Steam exclusively for PC. 2K, Activision, THQ, Bethesda, SquareEnix (Eidos), id Software, Gas Powered Games, Creative Assembly, Tripwire Interactive, FutureMark, Fatshark, Team17, Sidhe, Unknown Worlds. This makes these games completely Steam exclusive on PC. And specifically for GiantBomb if a game is Steam exclusive or uses Steamworks or not for PC should be pointed out straight on the 'general information' area for each game. This is somewhat vital information about the PC version of the game.
Also as a sidenote, people who use Steam seem to me to have a kind of console affiliation for the platform and will wait for PC games to land on the service, completely to the detriment of games that don't (a lot of indie games go here, and suffer for it).