Is MAPP gas actually hotter?
MAPP (which is actually a different gas marketed under the old name now) gives more heat than propane without supplemental oxygen, so it's still useable by people without training on mixed fuel torches. The fuel is more expensive than mixed setup, but the equipment cost is much lower; that's a reasonable trade off for something that isn't seeing constant use.

Speaking of which, MAPP torches will heat things up quicker than propane torches. It doesn't matter much for soldering plumbing, but it can be a real time saver if you're trying to loosen parts or heat something large. Some brazing compounds are also (technically) useable with MAPP, though the lower heat generally limits the size of the parts you can repair.

I believe there was a period of time where MAPP was widely used in cooling systems. The joints there generally need to be brazed rather than soldered, but the piping itself is easy to damage with high temperature torch. Cutting torches don't actually melt metals the way most people assume. Instead the torch heats the metal up to increase the rate of oxidation. Once the preheat is done a trigger on the torch allows even more oxygen into the flame, which reacts with the metal. This is why soldering requires flux, to prevent the hot fittings from oxidizing due to contact with the air. 

The point being that too much torch can be a problem. You can weaken things unintentionally, burn off the flux and ruin the joint, start the occasional fire. The main reason MAPP didn't completely die off is that it fits a somewhat useful niche where it's hotter than propane, but not nearly as hot or complicated as a gas welding/cutting setup.