After I wrote the post above, I noticed some new posts which appeared in the time I typed it up. I second Gary's point, that whenever an any soloist, instrumental or vocal, is performing with the arranger, the greatest shortcoming of such a setup is that everyone must follow the arranger beat. Many musicians are not used to this, especially vocalists. It can be cured by practice, but this is one difficulty I have accompanying "guest vocalists" on stage - I usually will switch to piano only, no arranger or drums at all, to do this.

An arranger player does not have the means to control the tempo finely - with my hands on the keyboard, I would need a third hand (or a foot) to keep on tempo controls. Perhaps another way to control it would be to allow slaving the arranger tempo to, say, the left hand chord speed - another suggestion for improvement in new arrangers . This may require a slight change in the player's technique, but with the player listening to the singer, he could then then control the accompaniment.