For what it's worth, I agree with you, I find the bass frequencies on even standard-range guitars to not be all that useful. In my own music productions, that part of the spectrum belongs almost entirely to the kick drum & bass guitar. However, once you cut out the low-end mud, the rest of the guitar sounds very good!
I've played a few gigs with it where I've been swapping between bass and guitar and for that it's been great - tbh the first time I did it I was just doing it to wind up the guitarist I was playing with as he was rather conservative about gear selection, and this was the embodiment of everything he despised!
I bought a stick years ago, but could never get to grips with it (I did a year on it, but was still very basic), and I guess the 9 string filled the gap for that sort of thing, without being unplayable for me... Plus I got it for a steal on gumtree here in the UK as the seller had got so annoyed with no shows he sold it dirt cheap.
I can't say I'm nearly as good as I'd like to be, but I've managed to learn enough to track some basslines and chordal parts.
For anyone new to the Chapman Stick, check out http://stick.com . It is a wonderfully quirky musical instrument.
anyways, the main reason to have extra strings is to have a greater range. if you just want to play very low notes, you can downtune a regular six-string guitar pretty far, you could just get a bass, or you could get a baritone guitar for something in between.
here's an example of a riff that uses almost the entire range of an eight-string guitar: https://youtu.be/v9RIJ4XlUPU?t=66