Most people buying full auto M16s (assuming this is a U.S. centric question) are doing so because those are collectors items. They cost upwards of $20k (edit: just checked and the cheapest one I can find is $31k, yeesh) and appreciate in value.

People buying non-NFA semiauto ARs do so for a multitude of reasons. If someone says it is for defense, then while the chance of using it as such is slim, if it does happen it is a lot more practical than using a bow and arrow.

For hunting, there are a lot of variables. For the most part intermediate cartridges aren't ideal for hunting, and in a lot of places they are banned for that purpose (at least for deer which are the most commonly hunted animal). You'd legally be required to use a full power cartridge much of the time. Bow and arrow hunting of deer is not something most people can pull off and it would lead to a lot more wounded deer running away.

I do think it's fairly common to use intermediate rounds on wild hogs, but at that point it's more about culling masses of them much of the time.