It is actually completely different than on Desura. While a game released on Steam, if the developers choose so, can be launched without Steam open and possibly have no copy protection whatsoever, a Steam release will still be tied to the Steam software for installation, which means I can't download and manage the game myself but will always have to go through the Steam software. This also means I can't download a Windows version when on Linux (or a Linux version when on Windows), which means I'm not free to have access to the software however I want regardless of operating system and software.
Desura, while offering a software which (I believe) behaves similar to Steam's, also offer the possibility of downloading the installer directly from the website; I can then download the version I want without needing to install any additional software (no proprietary client needed) and without having to access the service through a certain OS instead of my OS of choice.
While the ability to access any version of the software regardless of the OS may seem trivial, it still matters in practice to some (I keep all my digital purchases - all DRM-free - available offline for both Linux and Windows, since I use Linux as my desktop OS but I have to have WIndows on my laptop) and the inability of doing so is clearly a restriction of one's freedom in relation to how a user can access and use the software he/she paid for. For some (and I agree), this still counts as a form of DRM since it's the client (in this case, Steam's) that decides what I can access, where and when.
That said, I agree that the Steam copy can be shipped without copy protection; I personally see no reason not to release on Steam either with or without copy protection, but for a true DRM-free release there must be another service through which to access and download the game. The ability to choose the service from which to access one's copy of the game can be nothing but a good thing, as it gives the user the freedom to decide what he/she wants and what he/she wants to avoid (for example: Steam's auto-update and social functions vs ability to access the software in a full DRM-free way, see above).
I mention this here because, despite you not suggesting to use Steam as the only distributor (and I sincerely hope you won't see this post as an attack - it was not intended to be so, it was just meant to focus the attention on the differences between Steam's "copy-protection-free" vs "DRM-free" as I see it), I strongly feel a full DRM-free copy should be available and more importantly that the user should be able to choose to which service he prefers to submit to to access it.
For reference, there is a topic (which I did not follow) about Steam and DRM here: http://forums.obsidian.net/topic/61354-drm-free-option-why-not-just-make-all-steam-copies-drm-free/