What your rig did coming down the hill is what it's supposed to do. It downshifted to a lower gear to help slow the descent. If you just rode the brakes all the way down the hill and didn't take advantage of the engine braking you would at have very hot brakes at the bottom and have to frequently change your brake pads.
I keep my truck in T/H almost all the time. The front brake pads are original at 198,000 miles. The rear were changed about 170,000 miles.
The statement that shifting in to T/H is like putting your transmission in low gear is not quite true. If your just cruising down a flat road it should make no difference. If you're in a towing situation or a hill decent situation where you'd be better off in a lower gear than putting it in T/H may very well cause a down shift, as it should.