BUIS are a necessity for fighting gun. I had a catastrophic fail with my T1 during my class at CSAT. The front lens popped right off during our sighting is session. Without my BUIS I would have been screwed. If that were a gunfight, I'd probably be dead.
BUIS are relatively cheap and light so why not just have them on. Optics fail even high quality ones, I wouldn't take the chance if that weapon is one you plan on picking up when you going to fight. If its a range gun or fun gun then I say you can leave them off.