Dear All,
I have enjoyably read most of the posts in this thread. My two cents worth sheds some light on what I think an arranger is. As some of you might know, I have an xp60 as well, . I used the xp60 for over three years mostly to compose music from scratch or by totally warping midi files to my command. The one thing that was missing in all this was my voice. Even though I did produce some songs, my vocals and singing was always the weak point. How did I come to that realization?
I came to that realization when I bought the psr740. Gosh, after playing this instrument for a month now, my vocals have improved dramatically. I am able to stay in key(probably because of feedback from vocalizer), I sound fantastic (to my ears ofcourse). I heard some of the old recordings from xp60-only days, and the ones I am doing on the psr740, the difference is WILD!
How have my vocals improved so much? Well analyzing my playing a little bit, and after reading all the posts in this thread, I have come to the conclusion that while playing the arranger, I was assisted in experimenting with vocals as only an arranger can allow me. As someone said, the left hand becomes the band leader or orchestrator, and the right hand becomes the musician or soloist, I will add to that, and say that the voice/vocals become another soloist. So I have two soloists while playing the arranger, my right hand (which does what it pleases) and my voice which goes where it finds musical comfort. The only way that it was possible for me to have these two soloists FREE to experiment and grow to their full potential, was because of the strength of the arranger to provide a stable background, like the symphony orchestra provides to the improvising soloist (as another soul has mentioned on this thread).
To me the arranger is like a canvas, which is of one color. Between its monotonous repetition (one color), my two soloists twine their expression(colors) to produce an enjoyable musical piece(painting).
On the other hand, the xp60 is more like a grid, a graph with coordinates that I place objects in, ultimately resulting in a musical composition. The vocals (for me atleast) are added after the music is fully created, and then I cannot experiment, or there is tremendous rework. My two soloists are required to be disciplined and regimented, thus, loosing their freedom.
Alas, the arranger is but a tool, like the xp60, it has its edges, its idiosyncracies, and like the xp60 it helps me to accomplish a goal, although in a different way.

P.S. Still haven't got much use for the sustain pedal, I guess my brain hasn't caught up to that invention, just yet.