If you've ever taken an economics class you've learned the concept of an opportunity cost. Basically, it means what you're giving up by making a purchase, or in this case a sale. You choose to take an action because what you get for it is worth more to you than what you'd have otherwise done with it.
For me, the opportunity cost of trading a game in at GameStop is the game sitting unplayed in a case. That's not benefiting me at all. I traded in a bunch of DS games last week that'd I'd have never played again due to owning sequels or other, superior versions, and got $85 that I put toward my 3DS preorder. That benefits me quite a bit, and will allow me to buy more games with it than I otherwise would have.
GameStop may not be worth it to you, but, as far as I know, no one is forcing you to go there. You're free to do whatever you like with your old games, so let me do what I please with mine.