It is a very effective round if used as designed.
I've owned an AR-15 since 1969 and agree it is not the best caliber for hunting. My hunting rifle at the time was a 7 MM Remington Mag. Not exactly a plinking round but could be used on any size game from deer to moose. The expense of the ammo however made my AR-15 a more practical gun to use for practice.
I was never impressed with the 223/5.56 for terminal performance but at $2.00 a box of 20 rounds (ball ammo) army surplus at the time, I wasn't going to complain.
Fast forward to today and you will see that the AR-15 is replacing the old 22 LR as the gun most used for target practice and developing marksmanship skills. While I take out the 10/22 every now and then just for plinking, it is not the same for honing skills at targets 100 yards out or further.
Also you have to remember that law enforcement today is using the AR-15/M-4 as a upgrade to their hand guns in close up gun fights. From the use of them by SWAT teams it has spilled over to the patrol car and the officer on the street as additional resources to deal with barricaded suspects and keep the officer safer.
While I will use the 223/5.56 for varmints 65 lbs or less, I also have gone from the 55 grain bullets to the 60, 62, 69 and 75 grain bullets for more effective performance.
Is it the do all end all caliber, NO, But "It is a very effective round if used as designed"
Stay safe.
Jim