I think the reason we don't have a soft synth arranger yet is because of weak software. Why would a soft synth arranger need a super powerfull pc? The keyboard mfg don't use powerfull chips, but they are not running windows. What is required is an O/S dedicated to music and ONE piece of software that runs the styles and the voices. The software could be configured to use various buttons on a controller that are not strictly MIDI controls, so that you don't have to use the notes on the keyboard for variations, start/stop, etc.
I think the Ekco is on that right track, although it's not an arranger. If a few more msg jump in, increase the innovation, and lower the price, we will have what we want.
I haven't heard anyone say the organs on their keyboards are better then N.I.B4. soft synth. Makes you wonder.
Starkeeper