For awhile, I was also of this opinion, but after thinking on my personal habits and reading a few counterpoints from folks in the industry, I decided it isn't too bad. The fact that most folks take that trade-in money to turn around and buy new games is the big argument there. I personally probably wouldn't be able to buy as many full priced titles if I didn't turn in a few games here and there.
How many people take the trade in money and buy new games as opposed to buying the cheaper used copy? If you intend on trading it in later anyway, why spend the extra money on a new copy?
I read an article or two that made that claim. I understand it fully, many people want the latest and greatest software, including myself. Waiting for a price drop or used copies isn't something I am willing to do for the big releases.
Gamestop also offers more money when you use a trade-in towards a new game purchase, so that is one way that they stimulate new software sales.