My impression of the RepRap structure is that it is not capable of doing any serious machining, the lateral loads would result in significant deflection and thus error. Increasing the working envelope would exacerbate this. Machining foam would probably work OK, wood or plastic may not, and metal would be pretty much out. I have not manhandled one myself so this may be an incorrect impression.
you could probably tweak cut depth and feed speeds to accomodate different materials no? if you wanted to get haas-style cnc fancy, we'd have to get into tool changing, lubrication, tolerance and homing checks... all sorts of complicated goodness.
i guess it depends on what we want to purpose build this thing to do.