In any pictures I've seen of hands with extra fingers, the extras
weren't too good.  If you examine your hand closely you may notice the
thumb and four fingers are all different: they are each optimized for
certain kinds of work.  It looks to me like the extra finger comes in
lacking essential muscle and control systems; and it may be placed
poorly for any function at all.

But it's an interesting idea for the near future when our genetic
database is better understood, with technology for adjusting it.  I'd
be very interested to hear what a good violinist or pianist could do
with six-fingered hands.  

Cheers -- Martha Adams