I find it interesting how many comments refer to the used game market and how it hurts developers. One has to wonder, how does it hurt the consumer?
I spent a lot of time pirating games when I was in college, but now that I'm working fulltime, I buy my games. Unfortunately, I've been burnt several times. I was one of the poor bastards that went out and bought Age of Conan the day it came out, amongst a flurry of previews that said it was the best thing to happen to MMORPGs. Three days later, I cancelled my account and was looking for somebody to sell it to, when I realized that I couldn't -- the CD key was tied to my account, which was tied to my credit card; an obvious ploy to prevent me from reselling it.
Developers may see fewer sales when used resale is possible, but is that a problem with the resale or the developers? A good game is something that people will keep and play for quite awhile -- long enough that, if they do eventually decide to resell it, it will have minimal impact on overall sales. A poor game may be resold quickly and often, such that a single retail sale may result in several people owning the game. This would definitely have an impact on the developer, but that's the fault of the game being poor, not of resale. Preventing resale just inflates the numbers (by requiring everybody to buy a retail copy) and leaves a lot of customers with nothing when the game is a flop.
Preventing resale upsets the balance of risk, cost, and perceived value. Buying Guitar Hero World Tour at release wasn't a major risk, even though it sold for $200, because it had full (if not increased) resale value. I bought it myself, decided I didn't like it as much as Rock Band 2, and resold it for $20 less than I paid for it -- a worthwhile trial. If I couldn't resell it, the risk would have been much greater and I wouldn't have bought it on the first day -- I'd wait until somebody else with money to burn bought it so that I could try it out first.
Of course, you can also go into legal arguments, with the First Sale doctorine and how EULAs and DRM prevent our legal entitlement to resell what we have purchased, but laws are fluid things. The fact that resale is an important buyer protection is clear. If developers are complaining that it hurts their bottom line, perhaps they should be producing games that people don't want to resell.