They can provide cheap training for a primary defense 5.56mm AR. There are also competitions in which they shine. I like shooting mine in Steel Challenge matches. They have a rimfire rifle division.
I have a dedicated .22 LR AR. It's a blast to shoot and I built it very lightweight (3lbs, 5oz) as my gf complained that all my other guns were too heavy. She loves it and it gets her shooting a lot with me.
I suggest building or buying a dedicated .22 LR AR or at least an upper. The conversion kits are nice, but for .22 LR you want a 1:12-1:16" twist from what I understand. The 1:7" twist rates on many AR barrels is too fast to stabilize that little .22 LR bullet.