Squeak, you are confusing workstation use with arranger use.

If you already have a good workstation, you can easily control a rack workstation (although getting one WS's arpeggiator to reliably trigger another WS's arpeggiator can be tricky, amongst many other things). But getting a workstation or master keyboard to control a T2 for instance, is another matter altogether.

I can't imagine the complexity of mapping each and every control of my G70 to a master keyboard (and know full well that there isn't a MIDI command for every function on the front panel). Ergonomically, you could never fit all the needed buttons onto a module without making the buttons too small to be useful, live.

Primarily, all a rack WS module does is make sounds. You start to get into some VERY complicated country just trying to get master keyboard X to control module Y's arpeggiator, while stopping and starting module Z's RAM tracks, all the time pitch ribbon-ing it's internal synth engine. Most people just call up patch A from module B and layer it with another patch from another module.

Arrangers are SO much more complicated than this. Patches are the LEAST of your worries. Style selection on the fly, registration changes, multi-pad triggering, variations, breaks, intro/outro selection, tempo rits and accels, chord recognition modes, bass inversion, sync start and stop, need I go on?

What keyboard out there has the buttons and the flexibility to control all that, even if the arranger HAD the codes to be controlled? I know I certainly wouldn't want the job of setting the whole thing up!

I think you are coming at this backwards. It is FAR easier to control a WS module from an arranger, than it is to control an arranger module from a WS. Patch selection via MIDI is fairly commonplace in decent arrangers, the module can be programmed to do it's thing to the clock of the arranger, and a little fader/knob box can be added to control WS features that the arranger can't address.

Or is it that you'd just rather be SEEN playing a WS, while they HEAR you playing an arranger....?!

Practically, it's MUCH easier the other way.
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An arranger is just a tool. What matters is what you build with it..!