Many say the single-action is only suited for recreation and hunting, but it seems to me that if someone shoots them most of the time and likes them, they would do fine for home defense, largely because most people have more than one and in a HD situation there would be at least a chance to grab another should one be depleted. Why switch to a less familiar platform? Now for concealed carry or a car gun on a road trip, I can see where the reloading speed would be a big factor to consider. But, to repel boarders at home, I think a single-action that someone knows how to use and is familiar with would do just dandy. After all, there's a 12 gauge parked in the corner.
The main reason for a single-action is simply because they are...well, they are just "right" and life is too short to waste time doing things that aren't "right".