It's not just about the wheel holding up to the braking demands. The manufacturers have to think about the tire as well and how that seats on the rim under prolonged hard braking. We just did some testing (the torture test as I like to call it) where we ran the wheel at 35kph with continuous 17KgF braking power. 17KgF braking power is quite high and in real world situations you can't hold the brakes that hard for that long as you'll come to a stop, even on the steepest descents.
So, during the test temperature and tire pressure were continuously monitored and the test is to failure. The temperature of the rim actually plateaued around 107degrees Celcius and stayed there (ambient air temp was 29 degrees C), even though the braking forces were continuously applied. However this temperature even though it's not THAT high will start to transfer the heat into the tube and tire. The PSI inside the tire rose to about 168PSI (it was 105PSI when we started) and with the rolling and bumps the wheel was going through it was enough to blow the tire off the rim after almost 7 minuted of continuous hard braking. Obviously blowing a tire off a rim on a descent is a concern which is why brake modulation (on the brakes, off the brakes) is so important.
We ran another test of 4 seconds on, 4 seconds off. This test was 40kph with 40kg of weight on the front wheel. Braking force was 15KgF (again quite high for real world, but what you may do while braking hard going down a hill into a corner). This test was cool in that the temperature would go up and down and the max temperature achieved was actually a few minutes into the test, then the average temperature actually started to decline (it would still rise and fall, but overall the temperature was lower). This was due to the fact that the brake pads were brand new at the start of the test, and once they wore in a little bit the friction power was reduced. The max temperature reached for 4 seconds on, 4 seconds off was 68 degrees Celsius. After 15 minutes, the highest temperature reached was in the high 50's Celsius. So, you can ride down an hour long descent, just be smart about it and don't sit there riding the brakes or you an risk tire blowouts and such.
The main reason we (and others as well) have the two spoke count versions is looking at the long term demands a spoke goes through. There is a total load that goes into the spokes and when you add more spokes it transfers that load between them, so each spoke sees less of a demand. Since a lot of the people who are getting our wheels are using them as an everyday wheel we want to make something to where even after a couple years of riding the chance of a broken spoke is lowered. By adding the few extra spokes it spreads the load out more and it will also help with making a stronger and stiffer wheel, with only adding about 40 grams to the weight of the wheels.
The Handcrafted Revolution