There are exactly two possible reasons I'd ever sell a system:
1) The company's next-generation system is coming, has games I want, and is backwards-compatible.
e.g. I'll sell my Gamecube after the Revolution hits, or if I had had to spend my own money as opposed to being a kid at these various times, I would have sold my GB for the GB Pocket, my GB Pocket for the GBC, and my GBC for the GBA. (I do still have my GBA even though I've got an SP, but that's only because anything with frontlighting or backlighting seems to be completely unplayable in sunlight, whereas the original GBA is playable in sunlight--albeit more or less only in sunlight.)
2) I can get back as much money or, somehow, more money by selling it.
e.g. If I won an X-Box in a contest, I'd probably sell the thing immediately, because other than DOA, DDR, and possibly KOTOR, I couldn't care less about the system. But if I'd actually spent money to buy an X-Box, I probably wouldn't sell it even if I bought only five games for it ever; I'd either settle for the slim pickings I already like or, because I'd already blown my money, maybe I'd try to actually like the racing, sports, and FPS genres that I really couldn't care less for right now.
That's pretty much it. I tend to agree that I don't see how anyone can get rid of a system unless they've stopped liking gaming... The only thing I could support, I guess, is if a person didn't research the system's future release list before buying it and didn't realize they were throwing away money on a system that's absolutely and utterly devoid of any games or genres they care about. In that case, I'd say sure, get back as much money as you can and cut your losses.