Thats the point Mike. As a paying customer, when you get "allocated" without your consent, you got yourself a situation. I've seen it happen, its happened to me, and it will happen to you in time if you let it.
I suspect it hasn't happened to you yet because it tends to be tropical locations/operations where this occurs at from my experience. I know you spend a lot of time in cave country and/or colder locales where it doesn't seem to be as prevalent.
IMO, a paying customer should never be expected to look after any other paying customer, (other than their assigned buddy,) unless that paying customer choses to do so.
The problem i have with that is being expected to take care of someones elses customer just because i'm an instructor. I'll be there in a heartbeat to help out in an emergency if needed, but an operation that counts on that help, to do their job properly, is a slipshot operation IMO.