Guess I don't have much of an issue with it as well, I'm just super OCD and even though it sounds pretty petty but it bothers me to have my games in multiple places.
I probably should be more concerned on DS3 actually being a good game....
It's very convenient. For example, I recently bought a game I already owned, but I didn't realize I owned it because I got it as a DRM free version through an indie bundle and though I tend to DL the installers I don't check them before buying. I usually just check Steam, Desura, Amazon, or Gamersgate.
Of course, I'm in too deep now to restrict myself to one store and I like these weird crappy games that Steam won't carry. So I have to put in work to do an excel sheet now. Just checking my Steam library would make things much easier.