I'm totally guilty of that when playing bass behing a guitar track on my music - I rarely go above the 7th fret, and unless I briefly hit an octave or something, most of my bass playing is on the bottom three strings of my five.
That said, bass played solo is actually a rather expressive solo instrument, and when I'm just screwing around on my own I find I end up doing a lot of diad/triad stuff on the upper strings all over the neck. It's a really cool sound.
