Posting from the perspective of both helicopter pilot and USPA D-license skydiver:
Obviously, given that most on this forum have operated helicopters without doors many times, we all know it's pretty hard to simply fall out of an aircraft, particularly when properly secured in a seat by a belt or harness, even with the doors off. You pretty much have to want to get out.
That said, it is common practice in the US for pilots of non-cabin class aircraft used for skydiving to wear a pilot emergency parachute. This is not so much because they are afraid of falling out, nor are there regulatory requirements, but because there is a risk of catastrophic damage due to a skydiver exit gone wrong.
If I were to be dropping jumpers, I'd certainly wear a pilot emergency rig. And there's no question I'd be wearing a full skydiving rig if I was working un-tethered in any capacity in an aircraft dropping jumpers. Again, while it's hard to wind up in an impromptu skydive, sh*t does happen!