Andrew R Stewart 

No reason why a proper surface plate can't be mounted on a rolling base. As long as the plate is properly mounted it will be flat and most all the forces in aligning on a plate are vertical not horizontal. I have a wooden cover for my plate and it serves as both a protector when the plate is not in use and thus an additional counter/bench area. My plate isn't on castors but I have just over 500 sq ft in my basement working area (as opposed to storage and the garage solvent tank and mill). Here's a couple of shots of the basement as it is today. Note that some of the benches use 1" plumbing pipe as the structure. This is a very modular and moveable approach. I've had a serious work area in 12 locations (in 4 different states) and have moved this stuff a lot. Also note the South Bend lathe on the hand cart off to the side in one of the shots. My recent upgrade, still a few weeks from being able to make chips. Andy