It also doesn't help mileage, in fact it can make it worse on modern EFI cars.
When decelerating in gear, the engine injects NO fuel since the transmission is pushing the engine. If you put it in neutral and let it return to idle, it is injecting fuel enough to maintain idle.
The chances of damaging the transmission are very small, but keep in mind you're operating the transmission at 70 mph, but the pump is only spinning at idle.