"Can you explain why excactly it is not necessary to train the lower back through a full range of motion like other muscles? "
I found this paper Relationship between muscle fiber composition and functional capacity of back muscles... which says "A high percentage of type I fibers has been found in back muscles at the thoracic and lumbar levels. This is in accordance with the postural function of these muscles." "Type I fibers are slow twitch fibers that...are relatively resistant to fatigue." "Muscles with predominantly type I fibers...will be more involved in postural stability, sustained contraction, and endurance activities".
When they say "endurance activities", they mean holding your torso upright for hours at a time, with no rest. This is very different from how you use your biceps, for example. The argument is that, if the erector spinae are "designed" to provide stability and to have high endurance, then this is a good way to train them.
If you also consider that your disks are safer when your spine is in a stable, neutral position, then I arrive at the conclusion that the best approach is to train your back isometrically and in the neutral position.
Here is a caveat: I am addressing someone who just wants to be healthy and fit. However, if your goal is to be a trapeze artist, then that is a different matter. I once saw a video of trapeze artists warming up, and they go through a full range of motion. If your goal is hypertrophy in the back area, then that also is a different matter; consult a bodybuilder.
Hope that answers your question.