I don't usually carry an extra mag. My carry gun is for personal (including family) self defense, not taking out teams of terrorist. I have an extra mag in the car, and may carry it in the winter if I'm going to keep my coat on. I have never had a mag fail once in my life (I have had springs and a entire frame fail before, but there's never threads about bringing extra parts...).
It my option to be comfortable with 9 rounds of .40 or 7 rounds of .380. One mag with a lot of range time behind it is worth more than 4 brand new mags with no/little practice. Don't get me wrong, I'd love to be able to carry 30 rounds, however I find it more important to make sure when you carry you fill those extra little spaces in your pockets with:
1. A bright flashlight like a 4Sevens Quark or Surefire. Target identification is 1000 more important than an extra mag. I'd rather be shot because I held my fire if I couldn't identify someone in my house (or dark parking lot etc.) than accidentally shoot someone whose actions or identity I was misinterpreting. A bright flashlight that you know how to hold when you're also holding your gun can be more valuable than the gun in certain situations.
2. A cell phone. Being able to communicate with authorities before and after a self defense situation is essential.
Carrying a handgun is a big responsibility, however after a few weeks or months you will feel very awkward without your carry gun. I never thought once about carrying when I was single, but after getting married I realized how much you can love a person. I carry everyday because I must be prepared to defend our lives if someone thinks they can take advantage of us.
When I got my CPL, my plan was to carry a (relatively) large CZ P-40 and an extra mag. That lasted about a week before I realized carrying that much gun was totally impractical (in the winter no less). I couldn't carry if I wasn't wearing the right clothes in the winter, and I outright couldn't carry in the spring or summer. I could not take off my coat and it was extremely uncomfortable to sit. Two years later, and I now carry a P239 (.40) when I can, or a TCP (.380) when I'm wearing anything more than a swimsuit. Most people who claim to carry a full size 1911 and two extra mags actually own that gun, the two mags, and a CPL, but seldom mix them. Some people may carry all that every day, but I cannot. I know of 6 CPL holders and here is what/how they carry:
>I carry a compact when I can, but mostly carry a pocket .380
>Friend A carries a USP Compact every now and again, but it's too big for EDC.
>Friend B sometimes carries a 1911 in a backpack.
>Friend C switches EDC between a pocket .380 and a pocket .38 special.
>Friend D doesn't ever carry except for when he hunts.
>Friend E carry's a pocket .38 special in her purse.