Used games are definitely a problem.  They help add to the stagnation of progress we see in games as a whole.  More and more games are formulaic in quality and developers are afraid to stop making sequels and "Guaranteed sale of Shooter #174" because the used games take away sales from developers and scare them from trying anything new.

Everytime a used game is exchanged or bought it works in the favor of places like GameStop because they are the ones who reap all the profits.  They shaft you when you trade in by offering you an egregious amount of money for your games, then turn around and charge you 200% - 300% more for the very game that you just traded in.  They make truckloads of money off of this and the developers don't see any of it.  That means you aren't paying a game company anymore, you are paying a retail chain.

Why should you care?  Because if you like games, you want them to advance, and want more quality titles for the same price, then you need to support the actual company instead of supporting GameStop.

I really don't find too many well-thought out arguments that defend buying/selling used games either.  Most of them boil down to personal reasons and don't really explain the issue at hand; the company that made the product you are enjoying isn't getting any money from you thus they can not use any revenue from your used purchase to better their product.

I saw recently the marketing chief of GameStop made a comparison to selling your car doesn't affect the auto-industry's ability to sell cars (nevermind the sad and sorry state of the auto industry) but you don't trade in multiple cars at once, you don't purchase and trade-in your car every month or so.  This is a terrible comparison and shows that not even GameStop's marketing team can convincingly sell the idea of used-games being a win-win situation.  If there was profit sharing going on this would be a different story, but seeing how that isn't happening, this is a sad and sorry practice.

There are many things out there that we cannot afford but there is no "used" alternative to, and what do we do in that case?  We do without.  If a game is so important that you must play it, then you should do so at the retail price and support the company that made it, not some retail chain that is nothing more than a middle-man for getting you your product.