There is nothing in the SSA - which is what you agreed to when you bought the game - to ban the practice of archiving non-DRMed games from Steam and then using that game within the scope of your rights. While Valve may think they can and do regularly try, they are not permitted to arbitrarily make up rules as they go along.
What about clause 2.E?
Except as otherwise permitted under Section 2(C) with regard to the SDK, you may not, in whole or in part, copy, photocopy, reproduce, translate, reverse engineer, derive source code, modify, disassemble, decompile, create derivative works based on, or remove any proprietary notices or labels from the Software or any software accessed via Steam
without the prior consent, in writing, of Valve."
I'd say that forbids Steam users from copying files from a Steam game installation.
P.S.: It took me many posts, but I finally recognized your avatar. Cool one, :)
I wouldn't say that the support guy says they don't allow it, I'd say he didn't even read either of your messages properly. Those were pretty obvious copy/paste answers from a FAQ.
"As previously stated, you must have
the Steam application installed and a valid internet connection to install and play
the game on your computer. "
It may sound copy-pasted, but that answers my question negatively in my opinion.