To keep this related to mechanics rather than technique, here's what I'll say.
Your vehicle is attempting to displace some of the energy off of your brakes by putting extra force against the motor. This utilizes engine braking. Strictly, this does put additional stress on the transmission and motor that would not be there when in neutral. However, many modern vehicles will automatically cut fuel usage when the grade is sufficient to keep the motor running. This will save money by reducing gas consumption, and if you're in neutral you'll lose this benefit. The biggest difference mechanically is that stress is applied to the reverse side of the most of the gear teeth because the energy is pushing in the opposite direction.
So, to some extent (although possibly insignificant), yes it would theoretically cause increased wear on you transmission to have it in gear and utilizing engine braking when descending a steep hill, however, the trade off is putting a TON of extra stress on you braking system.
This part borders on technique, but it is important here: brakes will overheat. There is a very valid safety concern in using your brakes too much when gojng down hill. I believe this is why in many places it is legally required that semi-trucks are in gear on hills (if not at a times). It's also why lower gears (3, 2, 1, L or whatever) are available on automatic transmissions. I've seen brakes catch fire because of the amount of heat that friction causes, and it's really not a good situation. Yes, it is cheaper to replace pads and rotors than a transmission, but that's not the only thing that could sustain damage if things went wrong. If a brake did catch fire you're now looking at damage to the suspension, paint, glass, plastics, etc. All of this can very quickly add up to cost much more than a transmission rebuild/replacement. Additionally, when engine braking is happening, this also helps you brake faster if you need to, and it allows you to accelerate if you need to. More than all of costs concerns is the safety factor - is it worth the risk to yourself and others?
So in summary, there's really not much benefit to shifting to neutral (if any at all) and there are considerable safety concerns when not in gear. Plus you could save gas to some extent!