zombienerd, you don't just carry a spare for mowing down a crowd of potential bad guys, it is there for the post-event reload, the potential magazine failure, or the reload under cover. Magazines are pretty small compared to a full gun, they can be dropped in a pocket or you can get a belt-based carrier (I like my IWB one from CrossBreed).
Also, for those of us with different size magazines available, carrying the shortest mag makes for discreet CC, while the longer mag in the reload holder offers a big reload for very little effort.
RoostRider, it isn't a trust issue, it is an uncertainty in the conditions that might merit use of a CC pistol. One possibility is being knocked down, another is some damage to your pistol. It is about guaranteed that a potential SD shooting will not be conducted under the same conditions as you'd see at the range. Your wheelgun may be pretty damn reliable, but what do you do when you do have that unlikely
malfunction? What do you do when your carry ammo is half duds? (my reload is almost always either loaded with another type or vintage of HP's)
In general, I don't get the "reload in the car/truck" concept. If I'm in the car/truck, either it is now a large blunt trauma weapon, or an escape tool. If the reload isn't on your person, what good is it?