[captain]

There is aboutely nothing dangerous about using J valves if you understand how they work.
With the lever in the up position the valve will automaticly hold back between 300psi (single tank) and 500 psi (double manifold) in reserve. When the lever is turned down it releases the last 300 or 500 psi. You can leave the lever down at all times and use it just as a K valve.
Or you can start the dive with the lever up. If you get down to 300 or 500 psi it will become harder to suck air and also yor SPG will begin to drop with every inhalation. Flip the lever and down the last 300 or 500 psi will be available. Considering most diver don't breathe a tank down to less than 500 psi it will never come into play.
Also the lever must be in the down position in order to fill the tank. If the J lever is up in the case of a double manifold only the tank on the side without the J lever will be filled and the tank on the side with the J lever will not fill.
I have J valves on all of my tanks. They were developed before the SPG was invented and also for low or no vis conditions when an SPG can't be read.
When left in the up position it can also serve as a warning to a diver that is distracted and doesn't watch his SPG closely.