MOst professional published songwriters use ARRANGERS to arrange their songs. They hire producers and Arrangers who in turn bring their best musicians for that particular "style"

JAmes Taylor does not write all the parts for his songs

Neil Diamond does not write orchestrations for his songs

Many song writers use Arrangers. There are a few who can afford real people bringing real musicians. Then there are those of us who "hire" software arrangers withing hardware keyboards.

Thats what I use an Arranger for. As a performance instrument it's a toss up. We can record the tracks we want and play them back or we can take the keyboard and do the same thing making changes along the way.

Either way there is one or two people on a stage accompanying what may sound like 6 or 80 additional players. That part of the equation does not change. Whether on a CD or Keyboard.

As a songwriting toll the keyboard arrangers are priceless. As performance instruments a small step away from Karaoke.