It's economics - gearboxes just don't get rebuilt these days, and they're hard to re-assemble at the best of times. Even though you have probably just smashed or blunted one cheap baulk ring, you still have to pull most of the box apart to get to it, then re-assemble with an appropriate stack of shims, pre-loads and general hands-on experience with that specific box. As Type Rs are also noted for their tree-climbing skills, there's a good supply of bent ones - and that might just work out cheaper.