The problem with body armor is that it exacts a toll on the user. The more effective the armor, the harder it is for the wearer to adapt. Kevlar body armor weighs a lot and is far from breathable, so spending 8 hours wearing a 20lb vest can get quite tiring. Body armor capable of stopping rifle fire is oppressively heavy to wear, and a careful shooter can still exact a lethal injury by aiming for the head , groin,or armpit.
In the Trek verse of things, the situation is compounded because of the power requirements of an energy field. Making a nifty Stargate-type Goa'ould personal energy field means building a power source small enough to be portable, which would be tantamount to running around with a small power plant attached to your belt. I'd hate to be that guy if the power cell is damaged in battle -or if it overloads and goes boom!
We don't know how cumbersome Star Fleet security armor is. We don't know if it is weight equivalent to modern day Kevlar. And with super treknology, we might even assume it's much lighter and much more effective than any kind of real-world body armor.
I also agree that if this super security armor had power requirements, it wouldn't be more than a phaser power pack, which is to say quite small.
If the security armor was designed to be effective against Trekverse weaponry (disruptors/phasers) then the pinpoint lethal injury possibility seems diminished.
ST phaser-type weaponry seems to be broadly targeted against personnel, man-sized targets. I assume it's possible, but we didn't see pinpoint phasers targeting headshots or shooting legs out. Maybe it was solely the visual effects, but phaser beams seemed much broader attacking effect versus our millimeter-sized projectiles.