I have AR's in three different calibers, 5.56mm, 9mm, and .22 > Guess which one gets the most use? Couple of years back I am at the range watching people smoke through ammo. I notice that 90%+ of what they are shooting at is 50 yards or less. This is not rocket science, as they say, lets do some ammo cost math and figure this out!
My .22 AR dedicated upper is by far the most used firearm I shoot, and I have nine AR's in all different configurations as well as a 10/22 club in the safe. .22 AR's just make sense for a lot of people. As people point out, you can go to the range and blow through 500 rounds of ammo for less than $20. Do the same with your 5.56 and you have broken the piggy bank!
The dedicated upper can pay for itself in just a couple of trips to the range.