Maybe I'm stupid, by I always thought the purpose of the cowitness was in case your battery went dead, then you could still shoot with the BUIS without having to remove your red dot and lose your zero. Or in case the SHTF and you forgot to turn your red dot on or was turned off somehow or automatically shut down, then you could go ahead and use the BUIS without having to take the red dot off the weapon.
I could see that if you lost your scope. Everytime I look through the scope, I keep looking at the front sight and start trying to align the scope dot with the front sight and it throws me off. (I have a flip up rear sight). I may have to train myself to look through it differently.