My EDC is a SW40GVE and I carry it fully loaded (14+1) at all times, and I always carry at least one fully-loaded spare magazine. When you get right down to it, the vast majority of malfunctions experienced with semi-automatic pistols stem from either a improperly seated magazine, or a defective/broken magazine. Either condition can quickly turn your multi-shot master blaster into a single shot paperweight, and in the case of a defective or broken magazine, the only way to make the gun run again is to insert a properly working magazine. This is my primary reason for carrying a spare magazine; having a spare "part" readily available to get the gun running again in the event of a "break down".

Statistically speaking, most gunfights involve 3 or less shots being fired. That said, in today's complex tactical environment, we are seeing more and more cases where criminals attack in groups of 2, 3, or more, and in some cases, the criminals are dressed in body armor. In a nutshell, we can't afford to rely on statistical probability alone as the determining factor for how much ammunition we may, or may not need. Something I've said to my students before, and I'll repeat here; unless you are going swimming, there is really no such thing as "too much" ammunition. On the other hand, "too little" ammunition is quite quantifiable. This is my secondary reason for carrying a spare magazine; having spare ammunition available if the initial 15 rounds isn't enough.

I can't recall ever hearing a gunfight survivor saying, "I wish I would have had less ammunition with me".