Don't most of the WC DH riders ride with their saddles at full reach?
Nope.
As I said in a previous post, for proper dh riding you'll want it lower than xc height but not so low as you lose all control (keep that for dj and the bmx track).
On the dh bike I used to play around with saddle height a fair bit to get the best compromise, which I find is saddle at around the height of the stem (as mr.peat above), give or take an inch or so either way dependant on how tech the course is, eg. if its somewhere relatively non tech like say the cwmcarn dh then up a bit to make the pedally bits easier.
Each to thier own I guess, but don't try and tell people you can ride to your full potential downhill with your seat at an efficient xc pedalling position, it just can't be done, fact.
😀