“May be....but one thing they will do that an arranger won't...and that is, they'll all tell you exactly where you are as a musician”.

A person who plays an arranger keyboard and a person who plays an induvial instrument are two different categories. My Dad was a concert pianist; when he played my Yamaha he sounded awful. And when I played his piano I was terrible.

The person who plays an arranger keyboard needs many talents. I might say much more knowledge and talent, but it focused in many areas.
B
ad analogy, but I’ll give it a try:
A great trumpet player is like a man who is fantastic at first base, but no other position. An arranger player must have knowledge and ability in every position.

Just had eye surgery today, I hope this makes some sense.
John C.