Personally, if I can't take care of what ever the situation was with 16 rounds... I shouldn't have been in the situation in the first place (this assumes a situation where the rule of law is still in effect). I mean anything is possible, I won't deny that, but most of the actual carry usage I've read about have been against 1 or 2 aggressors. If I can't stop the threat with 16 (or even 6), then I really need to be back at the range practicing.
Now if this was an SHTF, then sure... I'd have all my mags ready, but that its a very different, and mostly improbable event situation.
There was a situation where an off duty officer was involved in a shooting at a McDonalds that was being robbed. Despite the outcome, he stated that being able to reload was very calming for him due to unloading his mag into the suspect and not being sure if there were accomplices in the parking lot.
In a defensive situation, you are quite likely to unload whatever is in your magazine, trained or not. You are also likely to miss the majority of your shots, trained or not. I realize this and therefore carry 2 extra mags for my Kimber.
I carry a 1911 with a eight round mag. Plus two extra eight round mags. With one in the chamber that makes twenty five rounds.
If I need a reload and drop the first one, I have another one to reach for. I train that way as well. The main reason for a extra mag is not just for the extra ammo but for a mag failure or a dropped mag as well.
9mm 12+1. Generally have a spare 10round in pocket. Alway have 2 10rnd And 1 12 round extra for total of 45. I don't think I will ever need them. Playing paintball taught me that the worst thing you can do in a firefight is run out of ammo (well 2nd worst, after losing of course).
Remember, the weakest part of the semi-automatic pistol is the magazine. It would sure suck to fire 1-2 rounds and have the base plate break on your mag.
I say carry a spare. It's just like everything else you carry, at first it feels unnecessary and heavy, but with time you stop noticing it's even there. In fact, you start to notice when it's NOT there.
If I'm carrying a 9mm, then I carry a spare 16 rounder for 33 rounds total. If a 45 (7+1) I carry a pair of 8 rounders for a total of 24 rounds. There's usually a .380 as a BUG with a spare as well.