vidsgame: A game may have Steamworks coded so far up its ass that it becomes very hard to remove it. Have a look at this wiki:
http://steam.wikia.com/wiki/List_of_DRM-free_games.
Gekko_Dekko: sounds reasonable. But some of these games were sold outside of steam on launch. If they included steam libraries too - it seems to be poor design choice, lol
I think the Steam libraries get added after the game updates. Like, let's say that Grand Theft Auto: San Andreas was sold outside of Steam on launch.
Rockstar for whatever reason decided to sell their game on Steam. They later decided to update the game. The Steam libraries could be required to even put the game on Steam or maybe Rockstar implemented Steamworks to make it easier to update the game. Now the game has Steam libraries in it. Now remember, this is all hypothetical but it does make sense.
San Andreas could run fine without the Steam client and Rockstar thought, our game is "DRM-Free" anyway. Let's see whether GOG would want to get in on our sales of the game and we could reach wider audience. So, all they have to do is give their copy of the game to GOG and start selling.
Most likely, an older game such as San Andreas or Spore will be all patched up, so everything would be updated. It most likely would be previously sold on Steam and no developer will touch a game this old unless but there are some cases where developers are nice enough to remove old files or even update the game.
When you're talking about Rockstar and EA. No way in hell are they going to bother, maybe with their newer games but that's it.