I used to fly 135 about 10 years ago, and we always had a couple of part-timers on the roster. One of them was one of our Check Airmen. He'd just come in to do checkrides and did very few actual trips - he was a full-time cop.

I would imagine most 135 companies have a part-timer or two. It gives them some scheduling flexibility without the overhead of too many full-timers. It also gives them (and you) a chance to "try before they buy".