First, it is not my reasoning. It is a standard of safe cave diving practice, brought into tech and rec. There is no reason to dive with a partially closed valve, and many reasons to never do so. "Back a 1/4 turn" is one of those pernicious urban myths about diving, that seems to have arisen out of nowhere, and gets defended to the death by people who are only repeating what they heard someone else say. But if you go and ask that person, they have no better idea of why it is done than the first person. (Why I hate the fact that diving instructors have no 'continuing education' requirements, and no minimum scholastic education requirements : Stupid and dangerous practices cannot be eliminated. And old wives' tales once 'learned', pass for knowledge. Even when there is no rationale for them. Even when they are complete nonsense.)
I know everyone and their mother got taught "open all the way, and back a quarter turn". I got taught it too. Ask an instructor why they teach it, and they will likely have some reasonable sounding rationale. But they made it up, or are just parroting whatever nonsense they were told. I parroted it when I first started teaching too.
A relevant thread is here:
http://www.scubaboard.com/forums/tanks-valves-bands/259240-newbie-question-bleeding-my-tank.html
Read what he did, and why the air flow stopped/slowed. Same thing happens with any partially closed valve. If the handle is turned back a 1/4 turn, if it is not opened to the stop and left there, it is a partially closed valve.
Another thread which covers the problems that a partially/mostly closed valve can cause:
http://www.scubaboard.com/forums/regulators/255179-hot-first-stage-3.html
I have seen it way too much. The DM (or the diver) opens the tank and cracks it back a 1/4 turn. Or do they actually close it and open it up a quarter turn (impossible to diagnose which is which)? Until at 60 feet, halfway through the dive, the reg that won't supply enough air (because the tank pressure dropped, and now the seat of the valve is not being pushed by 3000 psi but only 1500 psi, and so it restricts flow to the point that the diver thinks they are out of air.). Also known as "why I always carry a long hose regulator". Or, "why I open all the diver's valves all the way once we are on the descent line". They don't know that I am doing it, because it is behind their head. But I know the valves are open. All the way.
What tech agencies suggests diving with "open a 1/4" turn" valves? Did an instructor teach you this in a course? (I know the answer to that question already, I hope.) That is absolutely insane. Doing that means a rub on an overhead obstruction can cut your air off in a split second. IANTD, and TDI (I'm an instructor in both, though not current) emphasize absolutely the 'cave standard' from NSS-CDS: All the way open, or all the way closed. Period. For any number of reasons, from the recreational "Oops, did it backwards" problem, to the more serious "one rub on an overhang can cut off your air supply" problem.
Open the valve all the way. Or close it all the way. Either way you will know what's what by just taking one breath while looking at your SPG. And if it is tech, properly opened valves are less likely to be rolled shut by an obstruction.
The "back a 1/4 turn" nonsense may have once made sense with certain low pressure valves, back when valves used waxed thread as packing material, which could stop sealing if compressed by overcranking. But I don't think (though I do not know if)
scuba tanks ever used waxed thread packing. I know kitchen faucets and garden faucets used waxed thread packing. But then again a kitchen faucet is a regulating valve. A scuba tank valve is not. At least it is not designed to be.
