Given the price differential between 5.56mm and .22LR you could save a bit if you practice a lot.

There is also some advantage in noise. Even in rural areas where it is both legal and safe to practice the crack of a 5.56mm can cause concern and annoy neighbors. Using .22LR rounds, particularly if you stick to sub-sonic varieties, would lower your profile and make the difference between happy neighbors and a nagging feud.

On the other hand such conversions are not cheap and there are good quality dedicated .22LR rifles out there at similar prices as a conversion device. A separate .22LR weapon also saves wear and tear on the AR.

A possible advantage would be the savings in weight and bulk if you are in a situation where there are tight limits on what you can carry.