I carry several (cough) lights with me on patrol (private). Recently I've been patrolling a large business park which is under remodeling/re-construction. The whole place is fenced off but there is alot of equipment left overnight. They hired us when they had some problems with people hopping the fence at night.
For most of it I use my patrol vehicle that is fitted with a standard light bar with Alleys and such (my TK-40 is better than any spot on the car stock), but when I have to get out and go through some skinny spaces to the rear perimeter I take my Mag623, mostly because on flood it takes a nice picture to document a large rear area.
However it hasn't escaped my thoughts as I go into these rear areas and alleys that it would be very difficult to attack someone with such a bright torch (over 4K lumens, on a cool night you can feel the heat from 5 feet away). It would be a blind attack for sure.
And I have alot of other toys on my belt that I can whip out as the blinded assailant swings wildly.
It's not a good plan of defense on purpose, but if you happen to have one of these beauties on you, and someone does attack you in a dark alley, I say WTH, light em up.
For general purposes though you would be better off with pepper spray for real defense of a real attack (train to use the pepper and the light simultaneously, I do), but for those times that someone stepps out near you in the dark and you don't KNOW what the intent is and wouldn't spray a possible innocent, a good bright light in the face is a great way to say "You're in my personal space, what are your intentions!" (I wouldn't use the 623 for this though, It's just too bright to use to see what's going on with someone who, chances are is not going to be a problem. They would have to be real scary). I'd use a backup belt light (P1D over-driven slightly).
Remember you are liable for any damage you cause another individual that is not justified, and not just physical.
I have had several people spring out of the darkness at me and I invariably light them up. I don't know if it would stop an attack, but it sure stopped everyone I've ever done it to on the job, whatever their intent was. The lights used were all under 250 lumens IIRC.
The reaction is arms and hands blocking the beam and a cessation of forward movement (usually accompanied by "Hey don't shine that in my face"). If they were crazy, on drugs, or just out to get me I doubt it would stop them, but it would make their attack less precise, and more entertaining.
A scene from Crocodile Dundee comes to mind as I wrote this thread, modified slightly for the M*g623, "A flashlight?, THIS is a flashlight!".
A police officer saw my 623 beam the other night when I saw them trying to illuminate a canyon it was like a pinprick floating over a sea of dark. I fired my 623 down into the canyon which lit up even the hillside on the other end. The officer next to me just said "Holy Crap!!" I was almost embarrassed at how effective it was. It's seems inappropriate to have something so bright in such a small package..NOT!!
More practically something like the TK-40, even on lower levels is a great choice for those who work at night to stop someone in their tracks for it's large diameter modified spot type beam. Smaller belt lights are effective as well (especially if that's all you have with you at the time), but less so. And you have to be more precise with the beam that starts out the size of a magic marker.
A larger reflector such as most Mag-Mods and larger head lights like the TK-40 have a wider beam even on spot, and more effective spill. This is really important and not talked about at all that I have seen! They are much easier and much more effective in practice, especially with several individuals. You can "paint" them more effectively to keep them at a disadvantage less obviously as with a small head light (just innocently flick the light back and forth quickly as you talk). Usually a group will just look away or down as you talk to them. The larger head lights usually make better defensive blunt weapons also if it comes to that. I realize this is different for the average person who isn't uniformed and/or on private property, but some of the same techniques and rules still apply.