Well it's not always a rip-off. I've walked into Gamestop with six games to trade in and walked out with nine or ten of them. The trick is knowing how to manipulate their system. Very often they'll have deals like "trade in two games get an extra $5, trade in three games get an extra $10 and trade in four games and get an extra $20".
Most of the time these games don't have a minimum trade-in value so you could go in there and trade in four games you've no interest in playing ever again for a fair chunk of change. By doing this repeatedly you can build up a huge chunk of store credit without actually trading in very much.
Then you simply wait until they have a buy 2 get 1 free deal on used games and you go to to town. Walk in there and use your $80 or so to pick up a bucketload of their used titles that you just never got around to getting. One time I went in there with $100 in store credit, went to their bargain bin (I'd just bought a 360) and picked up damn near a dozen games.
So while it's not worth trading in most of the time (unless it's a brand new release) the system is meant to be exploited. Remember that their trade-in prices are based on things like how many copies they have had traded in so games can be devalued very quickly by quick complete / trade-ins.