It's not so much avoiding multiple purchases, although that's a concern too. It's also about stuff like--sometimes I don't know what to play so I just look at my list. If I have to go rooting around to find games, they won't be considered there. I only want to run one client, so I have no desire to buy things on something else with a client. I don't want too many companies to have my credit card (in fact, I use Paypal on Steam so even Valve doesn't have my info). When I format, I want to be able to redownload easily instead of remembering to go back to the other sites and redownloading.
As of right now I have an awkward "DRM-free indie game installers" folder on my computer from Indie Bundles having stuff. But from that GetGamesGo bundle some of them have DRM so some of those installers might not work.
And any perceived price benefit is phony; a game that goes on sale for $19 on D2D or Impulse will go on sale for $19 or less on Steam. Maybe not at the same time. Maybe not in the same sale. But if you're like most of us and you treat Steam like a "buffet" in that rather than actively intending to purchase X, Y, and Z you more see what comes down to the pipe and purchase as stuff gets cheap/appealing enough, having to wait for a spring sale for Steam to match the price from the winter sale on D2D or whatever is not a big deal.
Personally I also own all the consoles and I'm not really picky about control or graphical fidelity so if it's a game like ME3 which is Origin-works on PC, I'll just get the 360 version. It being Origin-works means I have no faith the DLC will ever go on sale, especially considering ME2's situation, so there's no price advantage to buying on PC versus on console.
Those are all fair points, though the majority aren't really a concern to me. I keep a separate list to track my depressingly large backlog. Clients don't really bother me much, Impulse/GameStop's doesn't even need to be running, but I only have a half-dozen games there anyway. I don't even own a credit card (though it would be useful to at least get a debit card, but eh...), so that's not something I need to think about. I keep back-ups of the installers on an external drive, which are culled every once in a while, when I've played the game and decided that it's one of those that I'm never going to touch again. Sounds like a bother, but in the end it'll be faster than having to re-download. The DRM'd installers could be an issue, yeah, but I'm not worried yet. GameFly gobbling up D2D is is of more immediate interest to me. I've backed up the games I care about, but we'll see if the online activation on titles they can't immediately migrate to GameFly is going to be turned off. I see no reason why they would, but you never know.
You are definitely right about games going on sale on Steam eventually. I've been holding back on purchasing more games lately (Holiday sale doesn't count!), the backlog is always in the back of my mind these days. I will make exceptions for games I feel like playing right now, but I'll definitely be slowing down in the future. The draw of dirt cheap games is ever present. "Why
not get it now?" has lead to some late night impulse buys... Nothing I regret so far however.
Ah, ME3. Not optimistic about that one, but I want to transfer my stupid save file (I know it's not going to mean much in the end, but I've dragged my goody-two shoes Shep through two games now...) and the PC version is going to be cheaper out of the gate (gonna get a physical copy from the UK). Wouldn't really save anything on DLC either by going console, since I'd likely need a Gold subscription to take advantage of any sale. Then there's the whole console shooter thing. I'm never really comfortable aiming with a stick, so I avoid shooters on consoles whenever possible.