In the Editor Attributes for a vehicle, there is a toggle to make it into a Simple Object to save performance. The typical setup, as in vanilla vehicles, is for the Simple Object version of the vehicle to represent a parked vehicle by default - wheel dampers at about 50%, all else pretty much stock.
Many CSLA vehicles have default Simple Object settings that diverge from this principle. For example, all helicopters have the blurred in-flight model for their rotors visible, and the Mi-24 has a muzzle flash visible. Many ground vehicles have their suspension fully extended, and some also have muzzle flashes visible. This makes it inconvenient to use the Simple Object setting, because the vehicles you use it on will probably look bad.
Also, for some vehicles, the Simple Object setting doesn't persist in the Editor - it resets after returning to the Editor from testing the mission.