It's a good way to save on editing costs, hoping nobody notices. TOS was never given tons of money to make episodes with, so all things considered it was a clever workaround.
The real bad news is, and this is a half-tangent, stock scenes of helm or navigator turning to look at the Captain often had Chekov or Sulu looking back when the other position is clearly manned by someone else entirely. It's highly surprising nobody took the time to do the needed updated reaction shots (quantity: two) that would be repeatedly used.
