Just sayin', but I think it is just a rule-of-thumb to replace certain items in pairs. Some items as a complete set. Pay up now or pay more later. And a safety issue comes into play as well.
- front shocks/struts
- rear shocks
- front brakes
- rear brakes
- front wheel bearings/seals
- rear wheel bearings/seals
- front tires
- rear tires
- all spark plugs
- all spark plug wires
- all fuel injectors