A perfect example of a model that has various screens is the Dell Inspiron 5000. The options are:
14″ 1024×768
14″ 1400×1050
15″ 1024×768
15″ 1400×1050
15″ 1600×1200
Using different aspect ratios is probably not done because it defines the shape of the laptop and backlight technology depends more on when it was designed.
Of course Dell is an extreme example because their laptops (at least when they produced the Inspiron 5000) were mainly built to order.