With arranger playing, I think it's about TASTE; knowing what sounds good. Since people of different ability can basically get the same thing out of an arranger, I think it's the player that knows which style to choose, which variation to use when and where, which style parts to re-voice to better fit his vocal style, which tempo is most appropriate for a tune, that sets one arranger player apart from the other. A decent right hand and the ability to emulate other (solo) instruments accurately, doesn't hurt either. I don't believe people with no basic musical talent will ever be great, or even good, arranger players. Arrangers are like any other instrument in the sense that, with a given amount of training, a talented player is always going to sound better than an untalented player.
chas
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"Faith means not wanting to know what is true." [Nietzsche]