If I play the Bass part manually, then the arranger stays OFF. If I allow the arranger to play - then I finger a chord with the left hand, but ALSO play the full chord/melody with the right hand.... just as if the arranger was off. I don't depend on the arranger top provide the feel... just the backing.(I almost NEVER use full kb mode for the arranger)
I don't play many melodies - I sing the melody. I hope this clears this up - I can't say it any simpler than that.
I admit that I use arrangers a little different than alot of other players, but I get alot out of them, and they provide me a tool, with which I can build upon to make interresting "live" arrangements on the fly.

Mostly, I rely on a simple drum beat, because I'd rather play the bass myself, anyway. I'v explored the strengths of the psr2k, and so far - it's best suited as my back-up band in 1)split mode - NO left hand sound.... or 2)split mode, MANUAL bass.