I see your point there but I'd argue that
a) two people using it at different times (so one person buys it, plays it, sells it) is no different from one person buying it and his whole family playing on it as well as him
b) if the game has no resale value then fewer people (as per Mammalian's post earlier) will buy the game new, as they can no longer hope to reclaim some of the initial cost - and those that do buy it will have less money to spend on further new games since they won't have recouped any cost by selling their old games on
c) having a flourishing second-hand market keeps numbers online up, making a better multiplayer experience. Get rid of second-hand buyers-->fewer people online-->fewer people stay online-->online gets rubbish-->no-one buys it new either
d) I can't justify spending £30 or £40 on a PC game; neither can many other people, hence second-hand market instead of buying on release. They're not missing revenue from us by allowing second-hand sales, because they're never going to get that revenue.
So IMO, they're just cutting off their nose to spite their face. All it's going to do for them is increase piracy and restrict the PC gaming environment. It's not like you can't sell your Xbox 360 or PS3 COD:MW2 second-hand, why do it to PC games?