You should see an arranger player with skilled left hand bass - you will want it.

The most dominant sounds in a full band is the bass and the drums. I think in arranger, bass is the one that will help accentuate certain parts of the song in a big way. Bass has such a dominant role that I hate to relegate that to the arranger. With a tight coordination of bass and right hand comping/improvisation, will create a powerful effects on a song and of course the right drum fills helps a lot too. For professional player of arranger, I just do not see how one can get away not playing the manual bass. How can you achieve tightness between left hand bass and right hand comping ? I think it's much easier with manual bass to achieve that.