I would like to clarify my point of view: when I was in Santa Barbara I spoke with Eric and he asked me how I liked my SD1. I replied that it was great but, having a 9000 pro already, he really didn't need another 76 notes keyboard. So the new XD9 could be the perfect solution for him, especially because he was looking just for new, more exciting styles and did not care much for all the sounds found inside the SD1.
The same goes for me: I like the 9000 pro for its sounds, and would be more than happy to find those sounds inside a module. In this way I would save also a lot of money. Right now, my only option would be spending all the money required for a 9000 pro without a real need for its styles or another 76 notes kb (that takes up a lot of space too).
Regarding the accessibility issue, I went to the point that I didn't place my two keyboards on a rack because this way the upper one would be less accessible and the lower one would have some of its features (buttons, even display) masked by the upper one.
I think that being able to access all the buttons and the display in real time without much effort is very important, otherwise you will never be able to really understand all the possibilities of your instrument. Of course, it's always possibile to find a good solution to lodge a module in an accessible way, maybe at a right angle with your keyboard, but I agree with Scott that when you play in public it becomes difficult to press the right buttons in time; you can always trigger your fills with a pedal, but how do you select the right instrument at the right moment? Sending a midi program change maybe? Hmmmmm......