My son has used the Supercard SD for Slot 2 for a few years, I'd say about 97% of the games on the DS are total trash that he plays for 5 minutes, gets frustrated with because of the horrible control or difficulty levels, and then never plays them again. For the 3% that he does get into and actually spends anytime with, I'll go out and buy to support the devs who are making quality stuff. Since there's no real way to test drive most DS games before buying, it's convenient to be able to download the ones he shows an interest in for a few hours, see if it's good or not, then either delete it or buy it based on that.
Also, it's nice to be able to use the game backups on these kinds of cards so that the game carts aren't constantly being mauled and dropped on the floor in the backseat of my car or at someone else's house, never to be found again. The slot 1 carts are pretty small, and if your kids misplace one, you're usually pretty lucky to find them again. This way, my son can access any of the games he owns and likes without switching out carts, and I can keep the legit copies stored away so they don't get lost.
Just posting this to let people know these cards aren't always about stealing IP from Nintendo or their 3rd party publishers. I know that's what a majority of people who buy them do, a majority by far I'm sure, but for some of us, the benefits of technology like this is, in my mind at least, completely legal and beneficial, and while I'm sure Nintendo would love it if I blindly ran out and bought DS trash like Foster's Home for Imaginary Friends: Imagination Invaders, I'd rather save it for quality titles like Orcs and Elves, Pokemon D&P, and Contra 4.