I've had this conversation with my father in the past and as an ASE Certified Master Mechanic, specializing in transmissions for 30+ years I take his work pretty seriously. In his professional opinion placing the car in neutral while braking won't cause any harm or abnormal wear and tear on the transmission. After all it is part of normal operation for the transmission. You shift in and out of drive every time you start/park your car, every time you back up etc. What it will do, as mentioned earlier is decrease your braking distance and help tremendously in slick conditions. Again, this is because you are simply stopping the inertia of the car rather than also fighting against the engine. This is assuming that you always wait for the transmission to fully engage into gear (this goes for D, 3, L, R, whatever) before stepping on the gas pedal.
However, during towing, you don't want any drive wheels turning. This is becuase you're forcing things inside the transmission to spin, but not supplying any lubrication. Under these circumstances it doesn't take long to do serious damage to the transmission. Therefore, if you're towing a front wheel drive, tow it by the front wheels. If you're towing a rear wheel drive, either tow by the rear wheels or remove the drive shaft to prevent the transmission from spinning. If you have a 4x4 you can place the transfer case in neutral (if possible) or it needs to be flatbeaded...
Moderator Car Audio/Video
2001 Black R/T
All the options
Toyo Proxes T1-S 245/45-17 (worn out)
Continental ContiExtremeContact 245/45-17
Brembo Front Brake Rotors
Hawk HPS Front and Rear Pads