Different strokes, etc.
I've never felt the need for more than the core four.
Know what you mean. Jaco didn't need 8 strings. And I thought why not sell my 15 basses and buy a 4 string P-bass? That's all I need!
And that's when I remembered that it was all I needed when I was 15 years old and only played one kind of music! I don't think I'm going back there...
Yes, I've got an 8 string bass (octaves) and I wish it were a 10 string. I looked at a 12 string bass (tripled strings) once and it sounded quite different again, but I didn't buy that one. Had it been a 10 string I probably would have. My 8 string is just a junker Carlo Robelli I got cheap at Sam Ash. But in spite of its cheapness it does have a great sound especially due to the killer electronics is has.
I really don't use it much, but to me it's a lot like my 12 string guitars (guitars, not bass...of which I own two). They are not what you play most of the time but when you WANT that 12 string sound, there is simply no substitute for a 12 string. How can you do Leadbelly without one? But as a main axe? I don't think so.
Same thing goes for my 8 string bass. As others have said here, an effects pedal (yeah got some of those too) is cool for certain sounds but simply can't give you that 8 string sound. In a way I'm a bit surprised that people (myself included) haven't done more experimenting musically with 8 and 12 string basses. Especially since there seems to be a certain (but not huge) popularity with 12 string electric guitars.
My "ultimate" which I would buy if I happened on a decent price would be a 15 string bass (basically a 5 string with trebled strings) And if I found a nice used 10 string I'd get that too. But for now I get by on the old Carlo Robelli. Yes, truly unique sound! And when it works, by God, it WORKS!