I intend to replace my rear differential fluid and gearbox fluid once the weather improves for the following reasons:

1, The oil is not cooled or filtered in anyway.
2, Due to the above, the oil will contain fine metallic particulate which will if left untreated accelerate wear.
3, Manufacturers are only interested in the car being reliable for the warranty period. After that, they couldn't careless as the type of people that buy a brand new car, are likely to do so every 3 years. (Here in the UK anyway).

For 30 minutes of your life and the cost of a bottle of oil, its gotta be a worthy investment.