There reasons I use Steam are:
-games are usually cheaper than most of the online contributor
-I don't need a cd/don't need to switch cds (a hard copy is nice for nostalgia... but I prefer the version that only needs virtual space)
-steam cloud, I'm not afraid to lose my save-states when cleaning up my Pc.
-Never had any problems/bugs(using it for 4 years now)
-great Library of indie games/a lot of games that are hard to find
-Auto-updates, when the update is available
-Faster delivery than buying online (Ebay, Amazon, etc.)
Yes, steam is also a DRM, but a DRM that doesn't keep me from playing. I only need to be online to activate the game or if I want to update it(or play online.. but well..that goes without saying)... but that isn't even necessary.. since it doesn't eat up my connection-speed/CPU and doesn't use more memory than any other messenger (unless it's updating, but auto-updating can be turned off).
I had a lot prejudices against steam before... it was pretty much the reason why I didn't bought HL 2 sooner... but now it's a big part of my pc-gaming experience.
And to answer your question, I wouldn't buy as many games from steam as I do now.. but I would still use it.