You do know that the Steam version does not require Steam to run, right? Steam is simply used as the download source.
It requires Steam to install, which is going to be a problem for anyone with limited Internet access, a banned (or locked) Steam account - or those preparing for the Great Zombie Apocalypse who want to avoid reliance on anything external.
And for those paranoid about security, having software able to download and execute code on your system without explicit authorisation is a no-no, particularly from a service that's yet to disclose full details of a previous security compromise.
