Do you know any game with uses the Steam Client for online multiplayer but run in single player without Steam?
Yes : Swords & Soldiers.
You have to install a seprate version which is completely without any steam if you want to play SP mode alone. And you won't get any "achievements" then, either. But at least the SP mode contains a "skirmish" mode nd so-called "challenges".
It's basically 2 different installations.
"Tiny and Big and Grandpa's Leftovers" is a similar gamewhich exists as an retail version completely without an steam, as I was told at the Games Com.
What is interesting, however, is that Steam-Games often get better additional stuff that Steam-less games.
It's like ... iTunes promoting iotself by selling music that's only available there
and nowhere else.