Cliff,
That is a great idea that I strongly support. That is one step between hardware and software based synth.
Jos,
your evaluation is faulty at best. Highend arranger keyboards are even more expensive than their synth/workstation counter parts. Furthermore, newer technology come first to high end arranger keyboards before non-arranger keyboards in a wide-scale distribution. Finally, VST is now a standard and there is no reason why it cannot be adopted by Highend arrangers.
This comes from a person who owns a studio based on Cubase SX and Nuendo 2 and use them extensively with High end arrangers.