People like Steam's community; Steam's feature-set (trading cards, Steam workshop for mods, launching from Big Picture Mode); Steam's sales ecosystem and the price pressure to get good discounts; Steam's infrastructure for cross-platform support for other OSes; and just not having to run multiple clients on their system. They can also be reasonably sure that Steam is not going to be abandoned by Valve, leaving their purchases hanging. I think given WP7 and GFWL (or even MS's handling of gaming software in W8--stuff like Galactic Reign, Gunstringer Dead Man Running, Zombies!!!, Rocket Riot 3D, The Harvest, etc.), people may have some reason to feel unsafe with MS--the response is typically "Yes, but this is the new thing! We're not going to abandon it".
Also worth noting: Developers can generate Steam keys 100% for free, so if MS did want to sell Steam keys on their own store, they could do so without giving Valve the 30%.
I find it hard to believe that if you actually wanted to know these things you wouldn't have been able to figure them out, but charitably assuming this is new information to you, now you know!
Are you suggesting that I was being disingenuous for asking? Perhaps I could have "figured it out" but I assumed this community of people who clearly hold Steam in high regard, would be as good a place to seek that information as any other...
Anyway, thanks for the info. It makes sense. the last time I played a game on PC, it was minesweeper. so no, I was genuinely unaware of the many of the benefits of Steam (other than its reputation for heavy discounts) nor was I aware of the options publishers have when selling games through Steam.
I hope however MS chooses to publish it, people adopt it en mass because it truly is great game, that will only get better with a large cross-connected community.