Most of the organized rides I've been on officially ban them, and I guess I've been lucky enough to pick ones where the support is so good people don't need them. I think the idea of centuries is to challenge yourself a little AND get away from cars. Personal sags defeat that.

My husband does a ride/race called the Everest Challenge, where, a rider can have "personal sag" but they must get one of the official signs for the car and you must be ABLE and WILLING to help any other rider in the same class as your rider.