Well, you would do that in order to have your whole crew reach the rep cap or save weight. You would leave the elite repair at UK sailor (not classed to any specific function like armament) until at least lvl 100.
So for having decent acc & rel while late-classing, you'll need to put on more than 200 vets. Oh, and it's advisable to put on vets as early as possible, as vet conversion success seems to be also dependent on sailor level.