[bmills]

A ring or sphereworld would indeed be massive, but don't forget that it would also be a dispersed mass. The gravity well would reach far out into the solar system, but would be rather weak at any given point. I doubt it would create a noticeable influx of matter.
To restate my original point, say you have a valuable asteroid. Would it make more sense to haul it directly to a processing facility in zero-g, or to use it to build a ring/sphereworld, and then have to mine it back out of the artificial dirt and expend fuel to haul it back out of the gravity (artificial or otherwise) of the world? The problem is not that it couldn't be done, but that it would be insanely uneconomical.