You could say the same of every new instrument ever. Why use a piano when a harpsichord is so much simpler to build? (Louder, mostly, but that's not my point. And pianos were much more expensive than any previous instrument when they were new.) Why build a harpsichord if you have stringed instruments? Why put strings on an instrument...why not just bang on that stick.

New instruments provide new means of expression not previously available. That alone is reason enough to keep experimenting and building new instruments, even if most of them never go anywhere or see anything resembling widespread adoption. They expand the palette of possibilities.

If you find a full keyboard strapped to a synthesizer more conducive to what you want to express today, that's fine. There are still guitars, and drums, and harpsichords, and pianos, and analog synthesizers. This doesn't close off those options to anyone, it only opens new doors.

Cost, or being "better" than some other option in some dimension, is not the reason for something like this. New means of expression is the reason, and it is reason enough.