I often play games while not online. For me, this is a key reason to not use Steam as in my last experience with Steam I had to be logged in or I could not play. So when I traveled, I could not play my Steam games until I landed and got logged in. (Some players tell me this is no longer an issue) However, for me, this was a deal breaker.
I am not a fan of Steam for a few reasons. Their forum seems to have a lot of members who run to the hysterical in reply instead of using contemplation and balance. This mob mentality often makes a great game get a low rating as many Steam members seem to lack the sophistication to appreciate sophisticated games. They seem to prefer simplistic game and often trash complex games, especially AGEOD games. This is a shame as these uninformed voices drown out actual discourse and serious discussion.
But I also prefer to house my own games and not have a company which has no obligation to me or to my gaming desires have a bit of a dictate over my games and the versions of my games. Perhaps things have changed at Steam. It has been a very long time since I used this interface, but I grew tired of not being able to use beta patches and having my updated versions revert to the Steam version.
Ok, I am a troglodyte, but why would anyone want an unknowable entity to buffer their game products while monitoring their usage? What is the value add? Convenience? Not persuasive. All my games are on my desktop. The thing is, Steam has no value add, just additional steps to get to game play, by my count, two more steps minimum.
As rwenstrup says in the original post, not all games run as well in Steam as when a stand alone. This is so true.
My take, own your own games. Do not let a surrogate service be a gateway to your enjoyment of games and gaming.