I had the right motor mount replaced on a 2005 Vibe base, 1.8L with 115000 miles on it. The mount and belt tensioner were replaced to solve a rattle that was most noticeable when the vehicle was stationary. In neutral or park, there was no rattle. When the car was put in drive or reverse it would rattle.
The rattle is now gone, but I wondered if it is best practice to replace motor mounts as a set instead of individually.