I don't see how trading in an old console plus 10 games you've already finished is a bad deal when they give you $100 for it. Fair market value for the games and consoles is near what they're paying you.
It may be an awful deal for people who are collectors, but the only thing my N64 and PS1 were collecting was dust.
Instead, I have an Xbox and Halo, which I've enjoyed more than my old N64 and PS1 games. So I think it really depends on your situation.
Can you mix and match on the games across systems? Since I have the PS2, I don't really need my PS1 which is, what, 7 years old already (bought the first day), but it would be hard to dig up 10 PS1 games. I do have many Dreamcast & N64 games also, so picking out 3 or 4 of each would be easy.