I wouldn't mind if it was optional, but apparently modding steam games is arbitrarily difficult. Since one of primary strengths of a TES game is in how easy it is to mod for, this would seem a bit backward.
This is patently incorrect. If anything, Steam / Valve has been very helpful to modders. If I recall correctly, Valve basically gave the OBSE team permission / the necessary information to make OBSE work with the Steam version of Oblivion. Something that Drive 2 Drive did
not do.
It's also a pain for serious modders, because we often keep several active installs at a time. I have about four installs of Morrowind at the moment - one for my mod, one for testing/helping with another mod, a base install to use as a reference, and an install I actually play.
Huh? I don't pretend to know much about Morrowind modding, but what would be the point of keeping several different installs running at the same time?
If steam gets in the way of that, it will be unacceptable. If it doesn't, and I can still mod at will, then fine - I'll finally sign up for a @$!# account.
I doubt Steam will get in the way of modding, though, as several have stated, everything is up for grabs as to what is and is not required on the PC edition of Skyrim. My uneducated guess is that Steam will not be required.