The co-witness capability isn't used all the time, it functions as a quick backup IF the red dot dies.
You normally leave your irons folded down out of the way, do a quick flip up if the dot goes bad.
You can continue shooting "through" your glass.
Without a co-witness capability, if you lose your dot your gun's out of the game until you can remove the optic and just revert to the flip-up irons.
Depending on the situation, your mount & what tools you may or may not have on you, that may take anywhere from a few seconds to the rest of your life.
I don't have a setup that blocks the optic on any of my ARs during normal use. The irons are either folded down out of the way or (as with two older carry handle versions) well below the glass.