I would have thought there were two bad points to this.
1, Gears/Engines must be more expensive to replace then brakes, surely you would be better served going through ten sets of brakes rather then one engine or gear box.
2, Surely using the engine to brake would cause the car to use more petrol as the revs increase, if your engine is turning over at 3,000 revs in 3rd gear, idlying in third gear, and you shift to 2nd gear and your revs jump to 5,000, surely that would mean the engine was sucking in more petrol, since every revolution of the engine requires some petrol to be drawn into it, of course without your foot on the accelerator this will be the lowest possible, but it would still draw some petrol and the more revs the more petrol it would draw.
If you just broke and then changed down gears, you could keep your revs at 3,000 the entire time and thus use less petrol.
Am I wrong in my way of thinking, am I missing some piece of logic that explains why you are taught to use the gears to slow down, or is this all some great conspiracy by driving instructors to have a great laugh at our expense....I think we should be told!