It's not that hard to get them off. Just make sure you brace the bottom of the crank against a piece of pipe or something else that is pressed against the ground, so when you whack it with the hammer, the force is tranferred to the ground via the pipe, not via the bottom bracket, frame, and wheels.

I recently repainted a frame with cottered cranks. I only removed the drive side one to cut the hassle in half. It works okay so far. The difficult part is getting them reinstalled correctly.