For me is not mandatory, because I would use STEAM anyway to launch the game. Because is where I have muy friends, where I can track my gaming time and where I have all my games together.
Whereas for me, it's just an online shop with some extra requirements, some HD/memory drain, and automatic updates (which isn't always a good thing. There are times I like to have control over whether to update a game or not).
Don't care about achievements, don't use any online "networking" sites (like Facebook, or twitter, or MS gamer profiles, or....), etc. Only thing I've used Steam for is getting Half Life 1, Portal (Mac), and Torchlight (Mac). And the only reason I have Steam in the first place is because it was installed with HL2.
(I much prefer purchasing games on physical media. For one, I deinstall and reinstall games all the time. Then there's the reassurance of having the thing in your hands. And finally, there's the fact that the US's broadband network just isn't Asia's - I just don't feel that downloading anything above, say, a few hundred meg is something that I can take casually. Some people may have networks where that's the case, but most people don't. Digital distribution's a nice idea, but the infrastructure just isn't there yet.)