There are lenses that "mean" to be shot wide open, like 300/2.8 for example. If you think a little, you will see that there is no point in having a 300/1.2 lens. So, when manufacturers construct a 1.2 lens for you, they actually "please" you at these apertures, but nothing more. When they make a 300/2.8, they are being seriuos.