There will be a problem. Steam automatically patches when you start it. If everyone else patches manually then you would normally keep the same version until everyone can patch there's. The person with Steam will find he suddenly can't play because Steam has patched his game without asking.
It's been a problem in my PBW games, but I suppose in those cases they didn't know to turn it off. I have Steam, but only for my copy of Portal. I made sure it wasn't going to effect SEV.