The problem with playing those odds is that you're betting with the thing most likely to fail and render your gun all but useless. The solution is so simple and easy that it makes no sense to me to go without a spare.
The other thing about the arguments against carrying a spare mag is that they always revolve around a gun fight. Sometimes things happen that just happen. I was sitting in the cruiser one time a few years ago when the base plate on the mag in my sidearm let go and pinged off into oblivion for some unknown reason and ammo went everywhere. Since I couldn't find the base plate I couldn't reassemble the mag, if I hadn't had a spare I'd have been SOL.
A more recent situation didn't even involve a failure. I had a couple of live traps set up on a guy's property to try and catch some domestic rabbits that he'd let loose. I stopped on my way to work at the hospital to check the traps and found a skunk in one of them. With no way to get it out of the trap except by dispatching it first, I took out my Kel-tec .380 to pop it in the head. Naturally, I was shooting from outside the skunk's range, about 8-10yds. Skunk heads are pretty small at that distance, I hit it on the third shot. Now, if I didn't have a spare mag I would have had to go through the rest of the day carrying a 4-shooter to protect myself against whatever might happen.
A spare reload is cheap, easy insurance that can come in handy even when you're not in a gunfight. Too easy to leave behind.
Of course, if you are in a gunfight, what are you going to do when the smoke clears and you're (hopefully) standing there with an empty pistol? Maybe the guy you just shot has friends or family that someone called and are on there way to take care of business. Maybe he's not quite out of the fight yet. Anything can happen, are you going to be ready for it?