Good point Bob.  My theory is that without etching as the final step, there was an increased risk of problems both short term (grinding debris) and long term (other material later deposited in crevices).  Smooth and clean is better than rough and dirty.

Sometimes the outgassed material forms a black coating on everything inside the holder - which would likely stop even the smooth etched quartz. 

The FT-243s are not the worst in this regard - there are other holder types from the mid twentieth century that seem to suffer even more - almost none of them work any more!  FT-241 is an example.  There is also a top-loading FT-243 variant that is bad in this way.