If you run through aggravating factors at sentencing as a checklist, then there's absolutely no reason for wearing body armor during commission to be a separate criminal charge, and thus no reason for charge-stacking.
That's actually MY point. The only real "aggravating factors" are previous convictions, and whether or not the crime was especially heinous. Assume an armed robbery conviction; the penalty is what it is. In order for the criminal to get more prison time (which he may richly deserve), the judge can't just say,
"Well, you've been an exceptionally bad boy, so I'm going to add ten more years to your sentence..." In order to get those extra years, there have to be extra charges brought, and convictions obtained. The easiest way to do this is to make things like using a firearm, wearing body armor, and wearing a mask or disguise separate charges. I'd have to assume that this is also the least costly for the taxpayer, since more things can get settled out in one trial.
* Thought experiment: come up with a circumstance in which wearing body armor would not be evidence of premeditation. I have a few, which I'll share later.
I'm sure I could come up with several, too. For instance... I have an aunt and uncle that are notoriously cheap. I mean... the kind of cheap where they refuse to leave tips for waitresses, they recycle fast-food drink cups...
We were in a decent restaurant once, and the bill came up to just under a hundred bucks. My aunt counted out the exact total of the bill, in cash, down to the penny, and gave it to our waiter (no tip included--I left some cash behind). Turns out she miscounted her cash, and shorted the guy a twenty. He caught up to us as we reached our car, and she paid him the rest. Officially, we could have all been charged with "theft of services" -- a misdemeanor. Since I CCW, though, it could have been upgraded to "armed robbery", and if I had been wearing body armor... well, you get the idea.
This is why I'm also in favor of letting the judiciary have some discretion in sentencing.