The problem as I see it is that a diver with no BCD training (and possibly no octo) and a dive buddy who only trained with a BCD using current training standards will not be on the same page when assistance is required and muscle memory needs to kick in.
Against my better judgment, I will again weigh in here. I actually (believe it or not) agree with this. However, it probably would be hard to find a diver (diving today) that has not had some form of BC training (even if they learned to dive a long time ago in a galaxy far far away). Probably easy to find a diver today who
hasn't had the old-style training - i.e., no BC; ditch and don; blow and go; buddy breathing with a double hose regulator (or even a single hose for that matter); free ascent from 30, 60 or 90 feet...
That said, when I choose to dive vintage (sans-BC, no octopus...) under optimal conditions and within my personal parameters (max 45' and a single dive), my dive buddy (usually, and if I even have one - no comments please) has the same old-style training (and modern training) I do.
On the occasion where my dive buddy has not had old-style training I make it very clear at the start that while I am perfectly capable of pulling he/she to the surface and getting them to shore/boat, don't look to me in an out-of-air situation. I have had divers refuse to dive with me as their buddy for that reason and I applaud their decision. As said many times on SB, it is all a matter of risk tolerance. The last insta-buddy gentleman I dove with had a pony bottle and was happy to see how I executed a vintage-style dive.
All that said, a BC should be used 100% of the time UNLESS one has the training and knowledge to dive without one, and only then under conditions and parameters maximizing the chance of a successful dive. While I am confident the views of others will be different, I personally love the feeling of freedom of diving without a BC. It takes me back to my childhood and the way I learned to dive (when diving was simpler it seems). My 2psi. Mark