Regarding the rubbish in the background comment, well it uses no CPU time and uses 37MB of RAM plus it's immensely useful to get matches with friends going especially on team games like left 4 dead where it's essential to have a system like steam to join games and lobbies and communicate between games.
I agree sometimes it's annoying when the steam update servers are slow or not online but those times are not common and as for cheating, well that's what you get for playing CS with noobs!
There are no privacy concerns with STEAM, where did you get this issue from? verify your email address and use a secure password and your account is pretty safe indeed, as safe as any other online account.
Also, set your apps to autologin on your PC, sure it's an issue if you share a PC with someone else but that's not STEAMs fault.