When you've got birds of different ages, stages, and genders, it's much easier and simpler to just offer one feed for everyone. The layer feed is usually about 15% to 16% protein, which is the minimum amount needed for egg production. But it's not enough for birds that are growing, or molting. Also, if you free range, or give treats frequently, you risk protein deficiencies which can lead to egg eating and feather picking. And the high calcium levels can be seriously damaging to the kidneys of non-laying birds.
There are just so many drawbacks for small flock owners, I've never seen the sense in feeding layer. As long as the flock has access to a calcium source, there really is no need for it. And most flocks produce much better on a grower or even starter feed.