IMHO some thinner hairs, like squirrel tail, have more action than BM. The shorter the wing (ie a small fly), the less action the hair will have. The sparser the hair wing, the more action it will have. An extremely sparse squirrel tail wing topped with perhaps 6-8 BM fibres to give it a little more visual bulk and 2 strands of crystal flash (hi-tech "horns") has been appearing on more and more of my SH wet flies the last few years.
Still stick to BM, however, for those reliable Lady Carolines that always seem to take up a substantial portion of my fly box.