"The question I have is "how many MG pilots, in actual practice, fly their MG like a pure sailplane, not putting themselves in situations where they would need to depend on their motor?"

It depends what you mean. I don't put myself in a situation where I don't believe there is a safe landing option - in 14 seasons of flying a motorglider the engine failed to start on just one occasion, as normal I had raised the prop on a high downwind to land in a field and I proceeded to land safely in the field. On the other hand when not in a competition I would sometimes set a shorter task or give up sooner and glide home if I didn't have the engine. In a competition, I wouldn't give up sooner without an engine, and with it I suffer from the higher decision height to give up and the higher minimum wing loading. Those are the disadvantages, the advantage is that you get home rapidly after abandoning a task, and don't experience the fatigue of a difficult or late retrieve.