As mentioned in the OP, BUIS cost and weigh little. Since Mister Murphy has jumped into my cargo pocket a time or two and picked some inopportune moments to make his presence known by fucking me over, thankfully never in a case where I lost an optic and had to swap to irons but I imagine that would be an unhappy face day. To sum up my stance here, I will take a quote from the mighty fine Pat Mac when he says "... Only and enabler, never a disabler." I always have zeroed and co-witnessed iron sights on my ARs, but I'm old and a creature of habit.
Personal experience, I've only ever had students or fellow shooters in classes encounter issues with optics and need to utilize Irons to finish up. So, for the training junkies out there I always recommend multiple sighting systems so you don't potentially wind up wasting time, ammo, and money on a class where you shoot degraded or not at all. That's classroom and square range life, never had it happen on deployment or have it happen to anybody I know personally on deployment.
Now, I'm not a purist either. I understand that "rail estate" comes at a premium and that when running a light, a laser, and an optic something doesn't make the cut. Totally cool in my book, 2 independent sighting systems and a white light for PID answers the mail in my book.
Case in point, the One Man National Asset: