[sigcurious]

So let's play devils advocate for a moment

* The AR-15's functional resemblance to the M-16 (it is a semi-auto variant) allows veterans, active military members and law enforcement officers to directly apply their professional firearms training to the protection of themselves, their loved ones and their community because of their vast familiarity with the recoil and position of the controls under stress.

Well then only police and military should have them, if they are functionally similar to an m-16, they're just as dangerous. Military and police have professional training to use such firearms.

* It's ability to change calibers allows the less affluent to practice more affordably, thus becoming safer and more proficient, through the use of inexpensive surplus military ammunition in 5.56 mm, imported ammunition in 7.62 x 39 mm or .22 LR ammo

* This same cailibery flexibility allows owners to save money by purchasing the same common ammunition that they use in their handgun, eg 9 mm, .40 cal., .45 cal, etc., ammunition

Great, so now these madmen that use them to kill people can practice doing it for less money and be better at it, while using military grade ammunition.

* The range of available hunting cartridges (eg .22 LR, 5.56, 6.8 and .50 BMG) allows for the taking of small, medium, large AND dangerous game, making it arguably THE most suitable firearm for the widest range of hunting and sporting purposes.

Nobody needs to hunt with .50bmg, they shoot airplanes and armored vehicles with that stuff. My <insert relative> hunts just fine with his bolt action 30-06 and .22 rifles, why would you need all that other stuff?

* The ability to change calibers allows owners to protect loved ones more safely and effectively since they can quickly switch between higher-penetration rifle-calibers (eg 5.56 mm) in order to protect themselves from threats wearing body armor (as some of the mass shooters have elected to do) or downgrade to a pistol caliber in order to reduce the risk of over-penetration ("know your target and what is beyond it!") in more urban/home settings

Now you think you're the police or rambo and have to have a rifle with you everywhere you go so you can shoot people wearing body armor? and if the 5.56 round penetrates so much, that just makes it more dangerous when the badguys use it!

(turning off the devils advocate for a moment, practically speaking if you have time to change uppers to adjust to the situation there may be a lot of better options available. Particularly since carrying a rifle full time in public along with multiple uppers and magazines to fit a variety of situations becomes impractical to say the least)

* The ability to mount an array of optical sights, lasers and flashlights allows shooters in general, and the vision-impaired, the elderly and disabled shooters in particular, to defend themselves more effectively in exactly the kind of reduced light scenarios that comprise most self defense situations.

Isn't that all the same stuff the police and military use to make their guns more deadly, now the bad guys will have it that much easier when they shoot up a school.

devil's advocate off...

While I applaud your eagerness to come up with answers to the question, as has been said before many times and implied by some of the responses already, it is not about need. Utility arguments such as yours, can either be twisted and refuted, or fall on deaf ears because the utility only exists for those who are already firearms users/owners.