Here we go again.
First let me say that I too am a WSI and also a Course Director and also as hbh2oguards described it, "head of diving operations at a large university."
My air is your air ... any time you want it ... no problem, no fuss, no muss. I have no concern whatever that I will not survive sharing my air with you, no matter whom you may be. I am far more concerned that I will not survive if you do not have enough confidence in your own skills and training to be comfortable to share air with me. In point of fact, if you will not share air with me (something that I insist on doing at the start of every non-operational dive, and even on operational dives if the situation permits) I will not dive with you. All those "rules" are nice, but sorry, the diving accident statistics do not bear out PB's concerns.
Oh ... how dreadful, how frightening, but what is the truth? In over a decade of work with the National Underwater Accident Center (where I was paid by NOAA, OSHA, NIOSH, the U.S. Coast Guard and later on DEMA) to investigate diving accidents) I never came accross an incident that reads anything like PB's description. Sorry, but it's made up out of his personal boogyman and does not partake of reality.
More of PB's personal boogyman, it just ain't ever happened. There were a few double fatalities back in the 1960s and 1970s that appeared to be the result of failed buddy-breathing, but never failed auxiliary use.
Basically good advice, unfortunately recreational divers today do not get the chance that we diving dinosaurs had to learn to do a real blow and go, the agencies do not permit it. So rather than have you try and get a 100 foot free ascent right on your first attempt, here ... please take the regulator that is in my mouth, I'll use my auxiliary, we'll settle down and calmly go to to the surface. There, wan't that easy?
I'm glad to see that we have some more common ground ... I could not agree more.
These rules have served me for 52 years of diving and well in excess of 10,000 dives.