Barry's on it. Case in point, on my M-4 that I took to Afghanistan last year, if I raised by rear BUIS, the red dot of my M-68 (Aimpoint M4S) rested on top of my front sight.
On a non-magnified optic, it provides a quick recovery capability if your batteries run out or your optic takes a whack that breaks something electronically.
The USAF has us sight each independently... M-68 with the rear BUIS folded down; then the BUIS by turning off the M-68, raise the rear BUIS, and sight in.
NRA Life Member