If someone's eyes are fully night-adapted, it doesn't take much light to actually hurt and cause reflexive blinking and flinching. As mentioned, just a quick flash will blind someone who is fully night-adapted.
I was at an IDPA match where we had a "night" stage, and even in low-light (for safety) and standing behind the firing line, a flash of light was dazzling.
A quick flash of light into the corner of a room can give time to ID anyone present, yet avoid covering them unintentionally with the gun muzzle, if the light is mounted on the gun; with the added benefit of briefly blinding them, if they are looking towards you.
We are all hoping, of course, that "they" aren't wearing NVGs...
