STEAM really doesn't seem that sophisticated; it does seem to do the job (though there are still hacks and cheats a-plenty in STEAM games). Initially I seem to remember reading that steam was going to have VFS, a virtual file system and a few other snappy sounding features that don't appear to have made it into the final product. Now it seems to be little more then a web based installer and wrapper -- but details are sketchy...
In any case I've never been to bothered by cheaters in FPS gaming; I'm not making a career out of gaming, am not a score whore that worries about where I am on the scored board, and if a server/game gets really out of hand as games sometimes do, if the game is appealing enough it will have fans that care enough and have another server going with a better game, or things will be cleaned up on the offending server the next day.
If Open Source brought with it a significantly greater security Linux and BSD servers wouldn't be the backbone of the "InterWeb"...
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