Honestly I actually prefer Steam over Impulse, if only because Steam is more convenient and Impulse is still pretty new to me.
I understand the feeling, but Steam is less convenient in one area: it must run for you to play your game(s). This is a constant irritation for myself and a friend, and it's why I don't buy Valve products, good as they are. It's like the cd-in-drive check, only it forces you to run it to launch the game so that Valve/Steam knows you're a verified user.
I recently had a computer crash and lost my windows install. I had Sins installed on a storage drive and while it did still function completetly Impulse could no longer recognize or update it. Steam on the other hand was easy, all I had to do was reinstall and copy one folder and my entire library of games including preferences, settings and mods now function exactly as they did pre crash.
Then why not file a feature request for Impulse? A "Point to Installed Location" right-click option so that it can test and verify the version, then handle it like it was just installed. It wouldn't be too hard to add, but you do have to ask! (Engineers don't always think about usability, but that's why users are so important.) One quick hash of the executable and testing against the list should reveal the version very easily.

I want to buy this game on Steam. Tell me why bringing it to a larger audience would be a bad thing. I know Steam is a competitor to Stardock's service. There are people that will use both services and can choose whichever. There are people who only use Steam. There are people who only use Impulse. I think having it equally available to all of these people would be much better in the long run. Sure there are those that would support Impulse over Steam, but these people are far fewer than those who won't even hear about it if it's not on Steam.
I'll agree and disagree with something here, most people "boycotting" Impulse do so because they think it is restrictive and just another DRM solution. If this is the case, then why would they ever buy from Steam when Steam requires you to login to even play the game you legally bought? Impulse may be a bit piggish still, but once a game is installed it's yours. To have to login to both update, install, and play with Steam just makes it a worse platform IMO. I only run Impluse once a week or so.
Would Stardock benefit from the added exposure? Sure! But when gamers are following games like Demigod and they see it distributed through Impulse, you'll get plenty of users interested in what the platform is. Valve is already very established so their growth is slow. But Stardock? They have plenty of space to soar upwards in.