Well, I've now done that with a vinyl couch (the cutting up part, I mean, not the "in the bathtub" part). Stuffing and vinyl in one garbage can, wood frame cut up into half-2x4 type pieces in the next, and once I find my wirecutters, the springs are the final load.
Partly, its because I don't have a vehicle that could take the intact couch to the dump, and partially, it's because I'm too cheap to pay to dump something I can theoretically throw away in the regular, free garbage. But mostly, it's the thrill of demolishing a couch to throw it away in dainty, garbageman-approved pieces. It's like getting away with something.