Basically, my point is that DIR is based on the use of a scooter. It dictates how you hold your light and how you deploy a reel. 99% of recreational divers will never use a scooter. Using a system that's built around a piece of equipment you will never use is sub-optimal.
Incorrect.
In DIR, what dictates which hand holds the primary light is the fact the diver is meant to donate the reg he/she is breathing in an out of gas situation.
For the DIR diver, in most situations, the diver is breathing the long hose which is installed on the right post. This is true for divers breathing backgas at the recreational, T1,T2, C1 and C2 levels. Recreational divers diving with single tanks are also taught to dive with the long hose/primary reg routing over his/her right shoulder as the idea is to be able to transition to diving doubles without significant changes in procedures.
The long hose/primary reg is installed on the right post so that it rolls on if the valve bumps against the ceiling of an overhead environment. Unlike the OOG diver, the donating diver has the ability to easily and quickly open a rolled off valve.
With the long hose/primary reg on the right post, the reg will route over the right shoulder. The natural hand to use to take that reg to donate to an out of gas divers is the right hand. If you the diver puts the primary light in his right hand he will blind the OOG diver with the primary light at the time of donation. As such, the DIR diver holds the primary light with his/her left hand.
Everything else follows from there.
Note: This is not to say that the DIR diver never holds the light in his/her right hand. For example, it is only practical to switch the light to the right hand to have a look at the compass or the spg.
