I think a lot of it is just that they use it sparingly. I know Mudvaynes bassist barely ever uses it unless it sounds right (like on Under my Skin) but he does tune GCGCF or F#BF#BE, which is already ridiculous! I do think it's just a case of, use when Nessescary. Protest the Hero's bassist is probably the best example I can think of. He uses the low string quite a lot but still knows when it wouldn't sound like rubbish. Also, forgot to mention dynamics. Playing a C on the Low B string creates a completely different feel than playing it on the A string, adds impact I feel.