I bought ED on Steam a week or so ago as I needed an Alt account to play with my son in the core systems while the "main" me is off exploring the Orion nebula
The way it worked was that after I purchased on Steam, it was Steam which gave me a CD key which I then had to use to create an account on the FD site (in order to play).
So - if FD were to give everyone "Steam" keys, those would generate CD keys which you could then use to create a totally separate (and free) account. Given that there's 350,000 copies of ED sold before Steam came along, that's quite a few free copies which would be sloshing about, so that isn't going to fly as most people will give them away.
What FD need to investigate (imho), is the ability to issue a code which allows people to add ED to their Steam library without creating (or being issued a key for) a new account. They simply then run it and log in with their existing (non-steam) account information.
I've never seen that done on Steam tbh, but it must be possible - just like when Steam have "free XX weekends" etc and games apppear in your library automatically (but not downloaded). All that needs to be done is to add a link on the Steam side to add ED to your Steam account without actually buying it, and that could be time limited I guess. If you don't have an FD account already, you'll still have to buy it and Steam would want you back through their store for that (maybe a slightly different launcher?).
So - rather than FD giving Steam keys away, I guess the question would be if Steam would let us add the game to our Libraries if we already own it. If people are using the same email accounts on both the FD and the Steam side, that becomes relatively easy I would imagine. If not, then an add-to-library code could be generated in the store page for previous purchasers (but not future ones most likely) and manually typed in.
But I guess we'll just have to wait and see.
TBH, as someone who owns both, it makes really no difference to me either way (but then I'm not bother about the Steam overlays, and I don't have any problems patching).