ShakaZulu:
Evertime I've trusted a "Circumstance Buddy" I've run into problems. ...
I completely agree.
When I am in the situation (rare) that I must dive with an unknown buddy, that is when I bring a pony bottle with me, enough to get me to the surface with a 3 min stop at 20 ft on my own.
These "buddies of circumstance" have a tendancy to disappear, and with them then goes your alternate air source (their octo) as well.
Anyone who is not highly experienced is probably only going to be a marginal buddy, at best. All you can hope is that they will stay with you, and share air if needed. Even a diver with a basic rescue class in their background will not have spent much time, other than a day, practicing rescues.
You would have to be lucky enough to be diving with a divemaster or an instructor, or else someone with several hundred dives to their credit (maybe), before your buddy would be a completely reliable asset under any circumstances.
That leads to the question, Why are today's divers trained so poorly? Answer: Because that is apparently all that the consumer public wants to pay for, apparently, with emphasis on "apparently."
How much does it cost to become a divemaster? At least $1000 if not more. Nobody seems willing to shell all that out, at the outset, for scuba training.