As to using a tablet with apps as a 'substitute' for a screen, it works in a lot of ways, but don't forget sometimes there are limitations if the apps are based on simple midi messages: for example, you cannot change the volumes of two parts independently if both parts are on the same midi channel. Some users experienced this problem with the Roland app for the Integra.
We are not talking about mere midi apps.... We are talking about using the tablet as a full integrated part of the keyboard... I.e. Replacing the screen of the keyboard.. So the keyboard has more room for switches buttons knobs and other controll elements....
Another example would be an arranger module, with a buildin ipad stand on the back... High speed connection, largeHD screen, yet not the production cost of such a screen...
Or even more thinking out of the box, imagine a laptop running V-arranger, with a controll surface that you place on top of the keyboard/touchpad .. Giving you a nice module with direct controll area...