Donny, who would those "great arranger keyboard artists" be? I'm not trying to be a smart-ass, but these "pickers" pretty much used no "assistance", in the form of loopers, sequences, effects, etc. They almost played "naked" I would think that the situation is different with an arranger, where the primary skill, or at least 1/2 the talent lies in manipulation of the auto features of the instrument and being able to adjust playing techniques to match the auto functions.
I'm not knocking that talent, but I think it's a different skill set than the guys we're talking about here.
On guitar, and on keyboards, too, I think as you get better, you tend to get back to basics. Most of the superior keyboard players I know don't use arrangers. Most of the superior entertainers I know use every possible trick. The "slippery" slope is the tendency to take the easy way out and allow the equipment, rather than the player to do more work than necessary.
Just an opinion. Take it for what it's worth...just my opinion.
P.S. I REALLY am interested in the names of those superior arranger PLAYERS. I'd like to watch them, too!