Scott,
this is a very interesting topic. Personally, since I perform in public only occasionally, I use an arranger as a compositional tool; should I lay down all the tracks manually with my Roland XV 3080, every song would take me two months and the drum track would sound...well, pitiful.
I agree that arrangers are unique, from this point of view, and I consider them like a vault of sounds and styles: that's the reason why I added the SD1 to my VA7: many more useful styles and more sounds to choose from.
Playing an arranger requires a good understanding of how a song is... arranged (no pun intended). Every song has to be built in a specific way and that's way all the arranger have different variations and it's up to you to experiment and to chooce the right variation (and the right instrument) for every section of your song (verse, chorus, bridge, etc).
This is a very interesting subject and one could go on for hours exploring all the aspects involved; for now I will stop here.