Firstly, no one has the right to question me on my "need" for a select-firearm/full automatic firearm. We don't question people on their need for an automatic transmission in their motor vehicle. A fully automatic firearm doesn't mean that said firearm operates in "god mode"(unlimited ammunition), so the positives gains from a full automatic firearm are not that great. Though, it isn't within society's scope of authority to question my "need", or even question why I want anything.
Secondly, a semi-automatic firearm can provide close to or the same rate of fire as a full automatic firearm(through "bump-firing", or other, "non-permanent" alterations). Still, though, one is required to reload the firearm in question. So, how exactly are "full automatic" firearms "more deadly" than semi-automatic firearms? I guess it is the same logic that is used to claim that silencers/suppressors make firearms "more deadly", when it's just as untrue. Though, I have learned(sadly) that in this instance, the anti-firearm crowd rarely, if not never, uses facts and truth to argue their points(I would rather have an honorable, and honest debate, rather than petty politically arguments which are totally unrelated).
Lastly, "full automatic" firearms are one of many types of arms that the Second Amendment covers. After all, the militia needs to be capable of fighting a war, if the need arises, and most, if not all, militaries utilize full automatic firearm, through select-fire mechanisms(safe, semi, and full auto fire options; though, not always).