The reason I carry a spare magazine is far less due to the need for a reload than for a need to replace a magazine due to a malfunction of some sort.
And THIS my friends is why I carry and advocate the use of revolvers for self defense. I can go on for days why but here are a few of my top reasons.
#1. Most modern semi auto's are incredibly reliable.
However, and I've seen experienced shooters do this at the range when they're not even under any pressure. Is they'll get a light strike or dud and they'll have that "knee jerk reaction" of pulling the trigger again, and nothing happens. You'll have to manually rack the slide, chamber a new round. Not so with a revolver. If in the event you get a malfunction with a revolver, you pull the trigger, nothing happens, it just goes "click", You pull the trigger again as a knee jerk reaction what happens? The cylinder rotates, cocks the hammer & the gun fires the next round. Unless tap & rack is a part of your daily/weekly training, it makes better sense to seriously consider a revolver.
#2. Semi auto's can jam on clothing if fired from undercover such as a coat pocket, revolvers with concealed hammers will not, such as my 642.
#3. And finally, raise your hand if you've seen guys at the range with their semi auto's rapid firing. I know more times than not, I've seen guys at the range blazing away. With revolvers, generally you shoot at a slower pace, theoretically (

) becoming a better shot as you gain experience.
My thoughts & opinions. We all differ, but no one here is more right or wrong, it's really up to you to decide what's best for your needs.