Hello DLeRium,
I own two AR-15 style rifles. One is an old Armalite from before the Clinton awb, chambered in the 5.56mm that the military uses, and a Colt that has been modified to fire .40 S&W handgun rounds.
The 5.56 mm and or .223 cartridges are not powerful at all compared to other rifle cartridges. Compared to say a .30-06 or .308 that is the most common round found in traditional deer rifles the cartridge that the AR-15 uses is very weak. The 5.56mm/.223 round fire very small light weight .22 caliber bullets at a medium speed. The rifles are loud as the round is supersonic, but they have very little kick to them. The 5.56/.223 round is a weak round as far as rifles go, and much of the kick of an AR-15 is absorbed by the reloading mechanism of the rifle. Anyone who complains about the kick of a 5.56/.223 round is just not ready to fire a centerfire rifle.
The military uses an M4 carbine that can be set to burst fire or in some cases full auto. If these guns have full auto then they can fire 750 rounds a minute. However the AR-15 is a civilian version of this, and will be semi-automatic. For those of you not in the know full auto means one can hold the trigger down and spray bullets, while semi-automatic means that one trigger pull fires one round. AR-15s can fire as fast as one can pull the trigger, a skilled shooter may get a round a second off with an AR-15 accurately. Having a full auto rifle that fires 750 rounds a minute is not much of an advantage over a semi-automatic rifle that fires 60 rounds a minute, as it is impossible to aim and hit with most of those 750 rounds.
The AR-15s are accurate rifles, not the best, but good. They are easy to shoot with very little recoil, and this leads to them being easy to fire quickly and accurately. They are, as you said, the Toyota Camry of rifles, good enough at everything.
As far as them being home defense weapons, the best answer is it depends on the round that one uses. The one I have that is chambered in .40S&W is an excellent home defense weapon, as long as an expanding bullet is used. The one chambered in 5.56/.223 is problematic because it is a rifle cartridge, and rifle cartridges will penetrate the walls of a house or apartment. Again the AR-15 is a particularly weak cartridge, much weaker than the traditional .30-06 or .308 that one would find in a traditional deer rifle, but rifles by design are set up to shoot over a long range, and so require a very fast bullet (I know there are rare exception like subsonic .300 blackout before anyone points those out).
My wife is very petite, and if she asked me to teach her how to shoot for a home defense situation I would start her out on a .22 rifle I have to familiarize her with shooting, and then move her up to the .40S&M AR-15 that I have. The AR-15 is a light weapon with very little recoil that is easy for a beginner to control. The magazine for it has 20 rounds, meaning that she will not have to deal with reloading the magazine in all likely hood, and if it does jam it is easy to correct. It is a rifle so longer than a hand gun, but for a beginner that is a benefit imho as it is less likely to be dropped or get out of control.
Contrary to the advice of our vice president, and others, I would not suggest to my wife that she try my double barrel shotgun for home defense. My shotgun kicks like a mule, and would be difficult for smaller person to control. It also only has two rounds, so it is more likely that she would have to break the gun open and reload it. Also, contrary to what some people claim, buckshot, birdshot, or a slug will go through walls.
As far as the popularity of the AR-style rifle goes for mass shooters, they are, as you said, the Camry of rifles. If you go to any gun store it is likely that you will see AR-15s for sale. The round they fire is just powerful enough to kill or seriously wound a human, but not so powerful that they have a lot of recoil that makes it difficult to control. They are easy to operate, accurate enough, and reliable enough. Hand guns are more commonly used for committing crimes because they are concealable, but if one just wants to walk into a place and shoot it up, and does not care about getting away, then why not go with a long gun? Concealability is no longer a factor. These kinds of mass shootings are still very rare, but horrific when they do occur. In general long guns are more powerful than hand guns, so if someone is going for maximum damage to make headlines, and dont care if they can get away anyways, then why not pick a long gun? I have a friend that likes shooting an older style semi-automatic rifle, the M1a1 that was used in WW2, and that would work just as well as an AR-15, but if you go to a big box gun store chances are they will have AR-15 style rifles and AK-47s, but not as likely that they will have other older rifles like an M1A1, or FN FAL, or more exotic brands of rifles.
As you said the AR-15 style rifle is the Toyota Camry of rifles now. So if one was going to go out and buy a rifle to shoot up a place it is the easy, obvious choice. In some ways the 5.56/.223 cartridge is not the best to commit these types of crimes, and it is not particularly deadly. The round that the older M1A1 uses is much more deadly. I dread the day that these kinds of criminals start using auto loading shotguns with buck-shot. If I was going to shoot up an enclosed room like a night club or school cafeteria, that would be my first choice. Although to be fair if one is shooting unresisting people who have their back turned to you and are just trying to get away, it likely does not matter which weapon one uses. It is easy to shoot helpless people in the back.