Although there are several anecdotes here from riders who have used road disc brakes on long steep descents with success, that doesn't necessarily prove that they are ready for general use. Manufacturers have to consider the entire range of potential users before they release a product; even if many customers are only 150 lb, the bikes still have to be made to withstand the 250 lb. riders who may buy the same bike - including those 250 lb. riders who may drag their brakes all the way down the steep descents.
You are quite right. At the meoment the road disc offerings are MTB based but on a road bike, and over time, not only will the calipers and discs be optimised for road bikes, but also the pad compounds will be as well.
I'm in the process atm, of putting on some 4 piston calipers on my road bike, because i want to experiment a bit, and I like the idea of a slightly bigger pad area to give me that increased modulation and extra reserve capacity.
I'm a bigger rider (96kg), and hence why i run the 180mm front disc as well.
I think it will be interesting to see where the road discs systems develop, and would not be surprised to see vented discs come back as I think that for road use, that is a better option.
Time will tell. There is a lot of development to be done though