People don't spend thousands of dollars on f/1.2 lenses to shoot them at f/8. The blanket recommendation to not shoot wide open is lazy advice as a replacement for actually learning how to handle a shallow depth of field.
Lenses have limitations, but that doesn't mean that certain settings are off limits, just means you need to expect limitations.
Well I have spent thousands of dollars on and 85mm f1.2L and rarely shoot it at f1.2. The DOF is usually shallower than I want for the subject I am shooting. However if I shoot that same 85mm f1.2L at say f1.8 I have often increased my DOF to where I want it and I already know that it is going to be sharper at f1.8 than my 85mm f1.8 I bought for sports and action shots at f1.8.
You spent thousands of dollars on the Canon 85mm f/1.2 L to not shoot it close to wide open?
That's strange. I wouldn't shell out any amount of money for an 85 f/1.2L. It's simply not worth it with the Sigma offering of the 85mm f/1.4
f1.8 is pretty close to wide open. I shoot for the DOF that I want/need in the composition. If it is wide open then that is what I shoot. If it is not then I stop down. But I can't get that razor thin DOF with an f2.8 or even a f1.8 lens. Kind of like when I was a boy growing up on the farm. Asked my grandad one time why he always wore long sleeve shirts whether it was the during the cold of January or the Heat of July. He looked at me and smiled a bit and said, "boy, I can always roll these long sleeves up and make em short, but no matter how much you try, you can't make them short sleeves of yours long ones."
Me, I'm from the camp of using the aperture that best suits the composition whether it is wide open or stopped down a bit.