I just had a bit of coop fun in DoW2! It is frickin' awesome. Truly awesome.
It's undeniable that GFWL and Steam simultaneously is so retarded. I'd say it's downright asinine. I really hope Relic decides to drop one and go exclusive with the other. I don't care which one they cure it'll be a hell of an achievement with global appreciation. Kinda like Cancer & AIDS.
For multiplayer GFWL is embedded into the game UI. It's how you join games and invite players to your games. It's not so deeply entrenched that it's a core part of it but I'm pretty sure with clever reverse-engineering it could be patched out.
Steam has no influence on multiplayer, it's just the DRM and update utility. It could have just as easily been implemented as GFWL has been or completely severed.
So to play online you only need a GFWL gamertag, but you need a Steam account to access the game. See the redundancy? Your Steam friends won't be able to join you, just as in L4D, but they'll be able to see that you're currently playing DoW2.
Now, here's the only benefit I can see of having both services crammed into the game: you have the choice for which one you prefer to communicate with. By default the GFWL voice channel is open for all participants, alternatively you can voice chat more privately via Steam's overlay (which also seems to have better network performance overall).
So that's the icing on the turd.
I'm trying to not let it bug me but it's such a nasty scar on what is an otherwise beautiful production.