Well y'know, I'm an infantry soldier. I've fired machineguns, mortars, heavy anti-tank weapons, pistols, most types of assault rifles, sniper rifles, grenade launchers - pretty much if you can lift it by yourself, I've pulled the trigger on it. I'm a qualified marksman, qualified CQB, and was provincial champion as a teenager. So I'm pretty comfortable with weapons.
I still vividly recall the first time I went down to the gun show to buy something to target shoot with. I got a beautiful 1919 Mauser made by Carl Gustav, mint condition. It was a true work of craftmanship, they simply don't make them like that anymore. Every single part had the serial number and the Carl Gustav crown stamped on it, by hand, to signify that it had passed Carl Gustav standards. I mean, the sight slider had the serial number and crown. The little lever that releases the sight slider had the serial number and crown, the firing pin, the sling attachments, even the screws had the crown (they were too small for the serial number). It was beautifully accurate too. I once bet a friend I could hit a paintball at 100 metres, and won. I loved it. So again, I'm pretty comfortable with weapons.
And yet - I also still vividly recall looking at some of the people at that gun show, looking at the stuff they were buying, up to and including belt fed machine-guns. (And this is Canada we're talking about. Despite what you may have heard, gun laws here are no obstacle to owning whatever firepower happens to turn you on). I saw a guy showing off photos of his high-tech arsenal the way most guys show off pictures of their kids. "This thing is better than porno," he said, referring to one of the eight (yes, eight) hunting knives he had strapped to himself at the time.
Why did he want this stuff? I dunno why, but as he was just about the weasliest, greasiest, ineffectual wannabe I've seen in my life, even before he pulled out his slide-show, so I'd guess some form of compensation for his total lack of effective masculinity was a key factor. I'd have handed my beautiful Mauser over to be melted down in an instant if it meant this guy had to hand his stuff over too. I would be totally unsurprised to read in the news one morning that he'd taken out twenty random innocents in his last pathetic attempt to get noticed. Or that he'd been caught molesting little girls in the playground. Or... well, pretty much anything. He was seriously creepy.
Of course Canada hasn't had a school-shooting incident in twenty years, so presumably he's still down in his mom's basement, masturbating over the latest issue of Soldier of Fortune and harming no-one, yet another upstanding example of the responsible gun owner. I've met a few others like him since then, and none of them have gone postal either. That doesn't make me comfy with the knowledge that they're out there oiling their glocks. Not one bit.
I'm not here to weigh in on the US gun control issue on either side - it's not my nation. I don't have a way to prevent the next school shooting. I do know some people want weapons for the wrong reasons.
You and I sir, are not only from the same Nation, but of the same mind. I've worked in firearms training since I was in my teens, and I've always held to the joke (it's almost not a joke any longer) that 50 percent of the Canadians I see using firearms are either completely unaware of how unsafe and unskilled they are, OR complete goofballs like the guy in your story, who live in moms basement with their SKS and other crappy Chinese made stuff available here with goofy folding stocks and fifty dollar red dot sights, and pride themselves on how switched on they are. Trying to balance business and vetting potentially "goofball" types from the training courses we offered at SigArms Academy isn't always easy. There are background checks done for certain courses, and some of them you can only take if you are military/police, or in some cases private security after being checked out.
You mentioned you competed in shooting as a kid - in what discipline? I was c-carded for the junior olympics shooting team for trap shooting back in the 80's when I was young, it was the first serious shooting training I received and participated in, and then I trained with Biathlon Alberta as well. Were you by chance into either of those?
Also, I agree with VonMessa 100%. I know everyone likes to break out the "tin foil hat" accusations whenever anyone talks about some sort of calamity, but I've worked in Africa, the Mid-east, and visited the Balkans after the troubles there, and worked with many guys who were present in Sarajevo during the first sieges there. I bet a year or two in any of these countries, before they were having to defend their family, drinking water, and food with rifles they may have made jokes about paranoid people too. It can't be overstated that when all of a sudden ATM cards don't work, the power is off, and the shelves are cleaned out at every store, how quickly people turn to thievery and banditry in order to secure the supplies they need in order to survive. Personally, I'd prefer to be prepared, and that requires having multiple high capacity magazines for every firearm you intend to use. The reasons given for ease of range use in terms of pre loading, as well as training/stoppage drills and the like are all extremely valid as well, but as anyone on this BBS who has witnessed or participated in armed conflict, having ready to hand magazines in quantity is just a given. Of course, you can debate for hours on the number, be it 4, 6, 8, 16, whatever - but the fact that more then 2 or 3 is required is generally agreed upon among armed professionals, at least with the primary rifle. Even a secondary weapon like a pistol, at least 2 extra high capacity magazines for emergency/tac/whatever reloads and malfunction drills are required. So, as to the OP question, the Ar-15 is primarily a close to mid range defensive tool, a very popular and effective one, and in order for it to be employed properly in a situation where one may face multiple armed threats at various ranges in various cover, with who knows what for armor and protection, a large amount of ammunition carried in high capacity magazines is absolutely essential.
This certainly isn't some new wave in the firearms world either. Look at how many extra magazines Bonnie and Clyde were found with, just for the Browning BAR semi auto rifles they had, a weapon of similar capability to the "Ar15", semi auto and magazine fed, and popular in the civilian world of that day. Yes they were criminals, but it doesn't change the fact that they were expecting to be in combat with their firearms, and were prepared accordingly for it.