The DRM (to be precise: the lack of draconic DRM) in SDC / Impulse is reason enough to prefer it over Steam. If a game was available on both I'd certainly buy the StarDock Verson of it.

The only problem SDC / Impulse has is that right now there are nearly no titles for it (that one actually would want to buy). GC2 and Sins are great but besides that there is no real interesting "modern" title. The next best thing is probably Space Rangers 2 but that one is a bit dated.
I really hope StarDock will get a lot of good games on Impulse fast (like: C&C3 or Oblivion or the KotOR titles or maybe some more strategy like Dawn of War / Company of Heroes). It's a little like gaming consoles: You can have the by far superior console, as long as there are no games for it it won't sell. So getting more (good, modern, anticipated) games for Impulse is imho necessary to be able to compete with Steam.

How about getting Guild Wars and maybe even Age of Conan on Impulse? I'd buy Guild Wars for sure and I'd maybe even take a look at AoC if it was available for easy purchase there.
Another game I'd instantly buy is Dark Messiah of Might and Magic (ok, that's probably not the biggest push to get market share ... but it is known and since it's a bit daten StarDock could probably get it on Impulse quite easy).

Generally I think it would be a great idea to get older games on Impulse. Those that didn't get them back then might be interested in doing so now, budget price combined with DRM-free and no installation hassle is a quite huge argument. I know I'd get a lot of older games I missed out on.