You hear this all time, especially in the US, from the EDC and gun communitys.
Unless you are active law enforcement, security or military, there is no logical point in doing it and you look like a tacticool mall-ninja.
The biggest points these people list for doing it is:
Magazine failure:
Entirely mute point. In case of a mag. failure in a serious self-defense situation, there is no time to perform a mag change. You are ****ed no matter what.
Also, with modern guns and magazines, such failures are so unlikely that it should be of no concern.
If you are carrying a gun, I believe it is your responsibility to familiarize yourself with it and practice with it on the range. The reliability of your magazines should not be called into question.
And last of all, IF you are so concerned with Mag. failure, carry a revolver!
More capacity:
Magazine capacity has more than doubled over the last century, with the power of the rounds commonly used in carry guns also having increased (.32/.380 ACP back then vs 45ACP/9mm today). I am aware that .32 and .380 remain very popular carry rounds to this day, however the majority of people carring a .32 are usually not the ones carrying spare magazines...
In reality, capacity and effectiveness was a non-issue back then and it is not a issue today. There is plenty of modern data that backs this up.
The notion of not having "enough capacity" in a self-defense situation is delusional, and borders on paranoia. Usually not more than 1 or 2 rounds are fired, very rarely more.
Are you afraid for your life? Do you expect to be assaulted by a gang armed with submachine guns hiding behind metal barrels in some abandoned back-alley at night? Real life is no action movie, but judging by what some of these people are carrying around with them, you might be forgiven for thinking it is.
Low-capacity carry pistol:
Seriously? If you are unsatisfied with the capacity of your carry pistol, carry a different pistol. The extra space taken up by one or two spare mags, would have been better used in carrying a bigger firearm to begin with. Also See point Nr. 2.
There are many more reasons listed by these people, but quite frankly most of them fall into one of the above or are outright ridiculous.
Carry a firearm by all means, but don’t be an action hero. In my opinion, the best one can hope do is to carry a compact revolver in .357 mag.
6 rounds is all you will ever need, and some even hold 7!
If you like to carry spare-mags simply because you want to and because you love guns and being over-prepared, then that’s fine. Just be honest about it and don’t try to justify it. There is no need to explain yourself, do whatever makes you happy.
If you are currently caring a spare mag and would like to tell be how I’m wrong, don’t even try.
Thank you.