I play in a three-piece where the guitarist/writer demos his tunes with him playing the bass.
he is just about ok on bass but often his lines are really predictable or ordinary.
The trouble is, if I deviate from what he has recorded, it throws him out completely! Even playing the same thing an octave higher will earn me a nervous sideways glance.
There are some songs where I get a bit more of a free rein, but mostly it is quite stultifying.
I'm really only doing it to keep my hand in, as my other band doesn't play that often, and almost never rehearses.