I agree that with a second set of brake levers on the tops the bars would be no different that a flat bar for descending, although I would want a pretty wide drop bar for this.

But I have never actually seen anyone using bar-top brake levers in a race around here. The CX guys usually just get off and run the muddy descents, or sketchily descend on the hoods with their weight forward. Again, they are probably trying to save weight like everybody else, and an extra set of brake levers, cables, and splitters on top of STI road levers is more, not less.

Still, the dual-brake lever idea is interesting enough that I might check it out next time I am at a shop that has CX stuff.