I think these days it's key to understand keyboard and non-keyboard controllers as a plurality of diverse input and articulation devices that can easily co-exist, mostly because they're complementary by nature, no matter which side one is leaning towards. It's definitely not a situation of keyboard controllers on one side and all the alternative controllers on the other side just as much as it's no sign of conceptual weakness that there are many different alternative controllers instead of just the one alternative to the traditional keyboard. I emphasize the latter because it's often being suggested that it has to be either the traditional keyboard or one other normative standard, the latter of which is often treated like one minority option due to its lack of traditional context.
But then, it already gets difficult where to draw the line, i.e. where a controller starts to become a non-keyboard controller. There are certainly a couple of alternatives that fall into a sort of hybrid category like the Haken Continuum or the ROLI seaboard, or the Touché by Expressive E, the latter of which makes for an immensely useful addition to a Minimoog as much as to a Eurorack system. What they all have in common though, is that they provide greater flexibility in terms of expression and enhanced controller modes of which MPE support is just one ingredient. So in that sense, I would say that there's a whole world of non-keyboard and enhanced keyboard-based controllers right now which shapes a continuum of alternative input devices that would include the aformentioned Continuum, Seaboard, and Touché amongst the Theremin, ribbon controllers, the touch stripes on the Prophet 12 and Pro 2, the KMI K-Board Pro, the Linnstrument, the Music Easel keyboard and its various reincarnations in Eurorack format (most notably Verbos and Sputnik), the Pressure Points, the infamous ROLI blocks, the iPad/iPhone in conjunction with a plethora of apps (from the Animoog to effect processing apps like the Eventide H9 control software), MPC-based controllers, Ableton's Push 2, and the planned SoundMachines Arches as well as its Buchla role models.
There are of course immense differences between some of those controllers, but they all provide alternatives to the standard keyboard controller and go either beyond the obvious expressive limits of the standard keyboard's on/off control and channel aftertouch, or the standard keyboard layout. And it's this plurality of options that I regard to be the strength of all these different controllers, as they help to customize one's controller options on different scales (pun intended) and for different needs.
The discussion about notes and scales is difficult and cannot be pinned down to the east coast vs west coast paradigms, as even Buchla established a standard for 12-tone scales with his 0.1 volts per semitone. That doesn't change the circumstance that the traditional and first and foremost organ-based keyboard controller is in itself nothing more than a contingent choice when it comes to synthesizers, and above that one of the most primitive ones, unlike the keyboard on an acoustic string instrument. And that's where the objectifiable advantages of alternative controllers come into play.
Beyond more control in terms of expression, the advantage of true non-keyboard controllers lies of course in opening a vast number of options beyond the 12-tone scale keyboard dogma, whether one is pursuing an aleatory or generative music approach or something else. As there is absolutely no reason for the synthesizer to be tied to any musical tradition, particularly all the Western music standards, basically all of the non-keyboard controllers more or less provide a solution for synthesizer-generated or rather electronic music coming to itself - granted, one perceives the synthesizer as an opportunity to transcend traditional boundaries in terms of sound and tonality.