I don't think I'd ever have a custom built. Although I think I know generally what makes up a bass that suits me, I know there's something about the sum of the parts and the way they work together that cannot be quantified or specified. I'm sure some of us have at some time had more than one example of what is 'the same model' bass but found they sound, play or feel slightly different.
I'd much rather find the bass for me by trying lots that have already been built, and upon finding one that feels right, adjust and modify it as necessary.
I'm 100% sure that if I specified a custom build that was on paper exactly the same as 'my bass', it would sound feel and play completely differently.