Seldom
I have no bone of contention at all, but I do believe that it is good to have all three services flying. It helps to provide contrasts, best practice etc.
Also, 1. Many of the Army NCO pilots would never pass the officer selection. (by saying this I am not for one second impugning the NCOs, rather the selection criteria)
2. There is no justification, in my opinion, for making all aircrew officers.