Why turning your scuba tank valve back 1/4 turn is unsafe

When I learned to dive I was told to always turn my tank valve back 1/4 turn.  This is no longer considered safe practice, but why?

The reason: A valve opened all the way and closed 1/4 turn is not usually dangerous, but a valve only open 1/4 turn could be a disaster.

How would you end up with a valve only open 1/4 of the way?

The scenario goes like this:

You open your tank valve all the way in preparation to dive.  A new diver you are working with, not realizing it is open, and who never learned "righty tighty, left loosey", closes your tank valve all the way and opens it 1/4 turn thinking he has opened it.  Clearly he had too much tequila or rum the night before.  On the surface during your predive safety check, you get plenty of air, your gauge reads full pressure and you go diving.  At depth you no longer can get adequate air causing a potential panic situation.

The simple solution to this problem is always open and close scuba tank valves all the way and so it is best to train all divers this way.

If the tank is all the way closed it will be caught while breathing on the surface.

Why did turning valves back 1/4 turn get so engrained in diving culture to begin with?

The answer is that early scuba valves and many industrial valves often jam so it was wise to turn them back 1/4 turn. Modern scuba valves do not require this precaution.  It is wise however never to forcefully open the valve all the way against the stop just in case.