But I'd want independent display & control of the pad from what's displayed on my monitor.
Really it's no different to having a second monitor on your system that just displays the image full screen without all the rest of the interface stuff that would normally get displayed on a primary monitor. Or having a TV hooked up to your PC so you can get a full screen view when working in Adobe AfterEffects or Premiere.
The advantage of being able to use a sideshow device would simply be that this screen is now able to be used as an input device at the same time.
One technique often used in Photoshop when using multiple monitors (as it gives you the space to do so), is to have a clone of your image come up filling the screen on another display so that as you zoom into one to edit, you can see how the tweak or fix looks without having to constantly zoom in and out every 2 seconds. A change may look fine when you're zoomed in at 800%, but blaringly obvious that it's been modified when viewed at 100% or "Fit to screen" and not fit in with the image at all.
Having the ability to zoom in and out and edit on an iPad (or similarly sized Sideshow touchscreen/pen display), and still be able to access all the palettes, and see a "fit to screen" view on the main PC would be very useful for me.
Essentially you could get a similar result with one of these tablet laptops with the flippy around touchscreen display...
...but with a couple of extra monitors plugged in.