Having used flashlights (as a civilian) for a dozen or so "close encounters", I strongly concur with Lightraven's and mclu2's thoughts.
Think, first, of AVOIDING trouble. By using flashlight(s) as a kind of tactical "early warning" system (searching the shadows where predators hide-in-wait) you'll avoid being surprised and often "preempt" some attacks. Now, that's just for starters.
(Warning: this gets kinda political. I've seen the movie, and I ain't talkin' Michael Moore's. No flaming intended here; no apologies, either.)
Survivors anticipate. They're never surprised, because they live in "Condition Orange" alertness (considered "paranoia" by the P.C.-conditioned pacifist Sheeple who preach Zero Tolerance for ANY weapons or "aggression" and live in "Condition White" let's-all-sing-Kumbaya-for-the-common-good/niceynice-think). That also means one needs to read/study the teachings of the great self-defense pistol-craft gurus, like Jeff Cooper, Massad Ayoob, and John Farnam, etc. -- and pay the price to (a) develop skills with certain weapons, and (b) cultivate a warrior Mind-Set and (c) integrate it all into your lifestyle... with the opinions of mommies, teachies, girlies, and rave-clubbies, etcetera, be damned.
A flashlight, by itself, is a poor weapon. However, when woven into your own "force continuum" (O.C./pepper-gas --> baton impact weapon --> Gun; knife back-up) the flashlight literally shines.
Every time I deployed a flashlight in a "crunch" scenario -- without showing any other weapon, I believe my body language ("command presence") strongly suggested that I was one mean MoFo with blue steel under my jacket. One is not born with such a demeanor, and societal conditioning strives to eradicate any whiff of it. However, waving some flashlight like Woody Allen waving linguini at a N.Y. taxi and hoping it'll scare a coven of Hells Angels on PCP twirling Bali-Song knives in your girlfriend's face is just a wet dream.
Flashlights can be fun techy-toys AND useful tools. However, like the immortal Dirty Harry said, "A man has got to know his limitations." As survival tools, flashlights have a key place... within tactical AND user-imposed limitations.
The one time I actually used a flashlight (a MAG-charger) as a weapon: I was first (civilian) on the scene of a fatal head-on crash... caused by a drunk who was hardly injured himself. As no cops had yet arrived, I began to direct traffic and then attempt first-aid on the critical victim. Then, the drunk-driver attacked me! One stiff gut-whack with the MAG made him "see the light" immediately.
By the way, when the cops and para-meds did arrive, I was the only guy with a potent-enough flashlight to guide the Flight-For-Life helicopter clear of power-lines and into a safe landing at an adjacent cow-pasture!