There is one race situation where discs make sense, fast and wet. That's a pretty UNcommon combination.
If you are racing a decent 'cross course, brakes just aren't that important. Just coasting will slow you down a whole lot. Updated mafacs are enough when racing on most courses.
As for the claim that it is 'easy' to build a lightweight bike equipped with discs, let's define lightweight. "lightweight" as in grazing the UCI minimum weight requirement? No. "Lightweight" compared to < $1000 bikes? Yes.
I'm not saying the market will not be flooded with disc equipped 'cross bikes because it will and at some point sponsors will require riding discs. Will the discs make a difference at most races? No.