It will. And I, for one, welcome our new Valve overlords. Every TW title since Empire has required Steam. Surely this is not news?
For multiplayer, I guess. That's suck a bit for me. My company provided laptop can run STW2, but Steam will surely be blocked by the security software. I really don't want to carry 2 laptops every where I go.

By the way, I don't have any TW games that come after Kingdoms. Don't have the XPack for Rome either (but I have 2 copies of Rome, which I played the least).