Interesting. Come to think of it, I think I never saw a bass player use one. Seems like it would be quite an adaptation in technique, for me anyway.I have recently embraced playing with a THUMB pick after years of not liking picks. When I play guitar I find picks uncomfortable and usually play with my finger and thumb (not that I am any kind of guitar player other than an occasional open mic).
Over the summer I bought a few thumb picks on a whim and voila, I love it. Not that I am giving up regular finger-style at any point, but I am pretty comfortable with a thumb pick.