I've only just spotted this topic. It has never occurred to that people really play to 'rules'? Surely it depends on the context, the song and the sound? I can think of a few nice bass lines that can't be played without an open string. Making use of the open string sound, rather than a fretted note to improve what you're playing surely must be the sign of a better player. As an example - how about the Fleetwood Mac song so beloved of car racing /Grand Prix TV programmes - play it without open strings and it sounds very odd! No open strings is as daft as saying no Bb's - you play them when they are appropriate!