Apparently is it for saving weight.
After re-watching the Lance shows I recorded (Lance Chronicles, Science of Lance, etc) I remember the Trek guy saying that the reason the mountain stage bikes don't have some of the same stuff done to their frames is to save weight. Going up a mountain is about reducing weight. Riding in the peloton is about drafting. TT is about slipping thru the wind. Hence the vast differences in the bikes and their setup. The Trek guy said that the 150 grams they save on the weight of the SSL frame is 150 paper clips - which isn't a lot but in the rider's mind when he's almost dead going up the mountain it might give him the mental edge. That's part of the reason the mountain bikes don't have the shark fin, etc.