I've been watching various YouTube videos, including PA4X76, Ketrons and Genos.
Not once have I seen anyone use the lowest keys and very seldom the highest keys in arranger mode. I can see where it would allow an extra split, but other than that that I question why you would need the extra keys, particularly the lower ones, in ARRANGER mode.
I have had a few 76s and I know I didn't use the lowest keys.
Just idle musings . . .