Exactly. Hair and scalp. Evolution already made a MIPS system. The thing lacking it was the test dummies that Virginia Tech uses, so now they recommend we put it in helmets too.
Or the ground being low traction: dusty/dirt/wet. Harder to control what you land on, but will diminish MIPS’ ROI in many situations.
Not having my hair and scalp act as the MIPS system is worth the $20 extra to me.