I give my girls a handful once in a while (maybe four times per month) as a treat, and it doesn't seem to do them any harm. This summer I was having a terrible problem with hawks, and my girls spent most of the day in the front of the barn so that they could at least be out of the coop, but they were still safe from the hawks. I have three outdoor cats, and I leave food out for them 24/7, and when my chickens were in the barn they would eat all of the cat food. It never caused any problems, and they looked and acted very healthy to me.
Some people say no, or only give it to them when they're molting, but from experience I think that it should be okay to give to them as a treat once in a while. Just dont give it to them all the time. Then it could cause problems.
-Ashley