Al least once. I'd have to dig up the article, but somewhere in one of the flat states (Kansas, I think?), a group of three armed people broke into a home that a teen was home at. He confronted and shot all of the robbers with an AR-15. I believe that two died on the scene, one made it out to the getaway car and bled to death in the car. The driver of the car was charged with three counts of felony murder.
AR-15 carbines and SBRs are very, very good for home defense, far better than a shotgun (long/unwieldy, low ammunition capacity) or handgun (poor sight radius, more difficult to aim), and the small, light bullet tends to not overpenetrate (e.g., you're less likely to accidentally shoot your neighbors than you might be with a larger, heavier bullet).
You know that AR doesn't stand for assault rifle, right? The AR is for Armalite, the inventors of the design. It's just a semiautomatic rifle with a detachable magazine, it's pretty common to use as a hunting rifle.
And yeah, you see AR-15 and it's workalikes all over the place because they're flexible. Literally the most common rifle in the US. That's why they're so common in public mass shootings - those shooters generally aren't buying a gun specifically for that sort of shooting, they're using a gun they already have access to or what they can readily purchase off the shelf.
It's not the best gun for any scenario, but it's a good enough gun for most and that's because it's modular and the guns and parts are both commonly available.
Basically never because they are ridiculously impractical for normal to carry around so they are virtually never available for anything to even think about using.