Let me try to explain what Donny is asking in terms of relating to the current synth/workstations (not that he needs me to explain for him).

Take the Korg M3 for example, this workstation has soft pads on it and it has a Karma algorithm in it that will actually play backing tracks. There are many sequences for many different styles of music. When you press one of the Pads and / or chords on the keys, the bass will follow you. It is not an arranger, but you can get some really good song results this way.

I have this feature on my Roland G6 workstation. There are built in arpeggiators that sound just like backing tracks on an arranger. However, there are NO intro or endings; unless you sequence them yourself. Actually, you can do fills and breaks by using multiple Arps and trigger then just like you do on a break.

The main difference between a workstation and an arranger is you have to work a lot harder on a workstation to get the same backing track song results. This is the bread and butter for an arranger, so it's much more natural and transparent.