There are a few of these types of alternative 'expressive' controllers
For example the Linnstrument
http://www.rogerlinndesign.com/linnstrument.html

I think that controllers like the Roli Seaboard and Linstrument don't get more wide spread acceptance because they require a keyboardist to learn a completely new skill - when they have already probably spent years learning to play a standard keyboard e.g. The Linnstrument would be more familiar to a guitar player

However, I have always been attracted to a keyboard based controller that would enable a keyboardist to play virtual instruments more closely to the real thing and do things like:

Bend the notes upward - like a guitarist does when he stretches the strings on the fretboard - say using polyphonic aftertouch
Add vibrato - like a violinist/cellist does when they rock their fingers on the neck/fingerboard - using left/right finger shaking on the key top
Add modulation - like a brass player does when they change their embrochure - say using channel aftertouch
Do portamento - like a trombone player does when they slide from note to note - by dragging your finger across several keytops
Change the articulation - add brass instrument falls or shakes etc - by finger slides or finger shakes