I'm actually on the fence about buying a new IWB holster with a mag "cup". As a daddy of two under the age of three, my pockets are usually full of any number of things, which for all our dignity I won't list here. Suffice to say, tossing a mag in my pocket is not an option- if you're going to do that, there shouldn't be anything else in the pocket, IMHO.
Real defend-your-life scenarios don't go according to plan, we all know that. When I'm out and about with the family, unless we're using a shopping cart, I'm lugging one of our girls- one is a solid 15 pounds including her car seat, and the toddler is pushing 30-35.
I consider an active shooter situation a lot. If I'm alone, my chances of running and safely escaping are not terrible. If I'm with my family, my chances of [all of us] escaping safely are drastically reduced. In that case, you'd bet I'd want the reload.
All that said, if you don't mind the extra weight/bulk, my favorite reasoning is short and sweet and to the point. Police officers carry extra magazines, wooden shoe?