Steam's DRM is why I've bought more games than I've pirated. iTunes DRM was why I've bought more songs than I would've pirated.
They were both more convenient than pirating the content.
Is a distribution channel a form of DRM?
And in any case both of those implement a very "weak" form of DRM in the app itself. What we're talking about here are hooks for "platform DRM" which is much, much more invasive. Nothing in Steam prevents you from copying files once installed, or even attaching a debugger and reading out game code and content. This will.
Which DRM are you talking about? iTunes hasn't had DRM on music for years. Like you said, iTunes being convenient is what makes it good. But iTunes is even better now that the DRM is gone (I can now play the songs I bought on non-Apple devices). I fail to see how you can argue that DRM doesn't make things worse.
My observation is that iTunes wouldn't have happened had they refused to implement DRM, not that DRM has merit.
Nobody wants DRM except those who hold power over content. In order to subvert them, we need to outmaneuver them, not refuse to play. In this light, iTunes is an excellent example of a DRM success story: fewer people with power over music content care to insist on music DRM.
Providers of content in other domains (e.g. books) will stop insisting on DRM when we demonstrate to them that systems without it are fundamentally more profitable in their domain than systems with it. I think this is doable, but not if we refuse to demonstrate it. They'll simply force people to switch platforms.
Steam's DRM has nothing to do with why individuals buy games on Steam. Steam offsets the net negative of DRM by bundling in a number of very attractive value-adds, like the ability to reinstall your games on any system without media, keeping all your games up to date for you, syncs settings and such (for Steamworks games) and provides an attractive platform for communicating between friends/arranging games and such. Not to mention that they have great sales. (For me, the first one is the killer feature. I hate playing the install + patch game.)
Now, they could easily provide all of those things without adding any DRM at all, but they don't, presumably because neither they nor their customers (which, make no mistake, are primarily the companies who distribute games through their platform) want it enough to cause it to come to pass.
I mean, I love Steam, it's incredibly handy and has singlehandedly transformed the way I consume games. But let's not pretend it's something it's not.
If steam had no DRM I am confident it would have been much easier for me to pirate it (I would have copied it from the friend who suggested it to me).
I went to pirate a film last year. Mind you, it was a film I had purchased but I didn't have it on holiday with me so I thought about just downloading it on a torrent site. But then I noticed YouTube offered the same movie for £2.50 rental, streamed on a fast connection in HD 1080p. That is what stopped me pirating it. Not because I couldn't find it, but because I found an option that was more convenient than waiting 8 hours for a download.
That's what Steam does really well. Why would I bother pirating that game, when I can pay a pretty small fee and have it ready to play within half an hour tops?
People below that line often want to pirate, to allow them to spend their money on other things.