I think other people have answered this appropriately, so I'll just add an example:
Just yesterday we drove eastbound over Monarch Pass in Colorado. The east side is 10 miles of 6-7% grade -- if you try going down that using nothing but your service brakes, you're going to end up on the runaway truck ramp halfway down. Driving in the mountains, the irresponsible thing is using your service brakes to maintain a fixed speed on the downhills rather than engine/transmission braking. Brakes fade under continued use and if you ride them to control your speed, they won't be effective when you actually need them to suddenly slow (like to make it around a curve) or stop the vehicle.
Oh, and the drum brakes that come on most trailers are much worse about fading than disc brakes - on my fifth wheel, I converted over to disc many years ago, and suggest you do the same if you think you're going to be using the trailer brakes on any long grades. Most trailers come with barely sufficient brakes for the hill country.
