Some folks like to descent the steep/tech sections in the drops. Katie Compton, for example, is somebody who uses them a ton.
I hardly ever used the drops other than going haahd on fast/windy/road sections. Set up your hoods right and you don't need to brake from the drops* (*note: unless you plan on winning 15 National Championships in a row).
Tradition matters too. UCI banned them I think after Thomas Frischknecht came in 2nd at worlds in 1997 on flat bars.
Frischknecht won 2nd in a 2-up spring against a drop-bar clad Bramati. Interesting also to note Adrie van der Poel was 4th in this race.
Spinergy Rev X wheels, flat bars....It was all to much for the UCI. Le sigh.
These days flat bars most exclude themselves. You can't make a cx bike fit properly (i.e. put the wheels in the right place) with them.