I can't comment about downshifting the automatic. I would suggest referring to the owners manual for specifics. I am sure it goes into great detail.

As for down-shifting a manual transmission, I own a standard in large part so I can downshift the transmission. One benefit of downshifting is to slow the car as you corner, set the car up for the turn, and position the engine to accelerate out at peak performance.

Another reason is to simply slow the car down.

I have never had to replace a transmission or even do clutch work. And no drive-line problems either. Every driver is different.