This is America damn it and we are Americans! We don't have to need a rifle- or anything else for that matter- to buy one. We buy a rifle because we want one. Sometimes we want something that suits our needs well and sometimes we want something only because it scratches an itch. Be honest with yourself and get the M&P-10 because you want it. If you have to justify "needing" it you don't really want it. In that case pass. You're welcome- always glad to help a fellow shooter gain some clarity

I am satisfied with the M&P-10. I stopped worryng about recoverng brass while hunting. Even with my bolt action rifles I rarely recovered my brass after the shot.

Unloading an AR in a tree stand will be more awkward. Removing the mag won't be a problem but clearing the chamber without dropping the round won't be as easy as a bolt action. Only you can decide if it's too much trouble or not.

308 ammo isn't difficult to get and if the price of the rifle leaves you without enough funds to shoot don't buy it. Can't help you with the southpaw thing. I'm right handed.

Bottom line- You don't need an M&P-10. If you want one and it suits your purposes get one. If not- don't. So far mine has been a good rifle and suits my purposes just fine