I find my rims scratch, crack or get dirty more quickly than the tires. Rims will probably not be as round or balanced as they once were after using a whole set of tires on them. I always get new rims if i get new tires (and they are usually the cheapest part of a new set). It would be so hard to remove all of the rubber from a used rim without destroying it. The old rubber will make the new rubber not sit flush so would make you loose grip.

If there is any reason you might want to reuse rims (special ones or something) then you probably have to file the rubber off. I did buy some carbon rims once, they were black, rock hard and spoked. They were pretty good (apart from the tire on them was crap), but i could not get the rubber off properly. If you glued the tires on properly then they should be very hard to get off.

I recommend just getting new rims, its not worth the effort, and will give you better performance. I do recommend reusing the inserts though. They dont get bashed around as much and can be reused a couple of times. They can get deformed, but the moulded ones are generally good. Still after a while you should replace it.

Just my 2cents