Corn is the first ingredient in most commercial feeds because it is CHEAP, not because it is the best choice. Same reason corn in some form or other is a top ingredient in most processed foods in general. Corn is a cheap carbohydrate source. Straight corn fed to chickens should be viewed IMO as a treat/candy ect. Blended into a balanced ration, is different. Scratch mix is not meant to be a primary ration, it is a treat- so yes, everything in moderation. They don't need it, but boy to they like it. I tend to say corn = junk food, as I often come across people in person that think scratch grain = chicken food (and that is all they give). Most of you guys know better than that, sorry.
Yes, the original farmstead dual purpose chickens ate grass, weed seeds, left over buggy grain, various bugs, scraps ect- and likely lived a few years and went into the pot or got eaten by predators. They laid eggs for a portion of the year, broody was good, they did not usually lay year round. The original wild fowl lived(s) off of greens, seeds and bugs. These guys also did not lay a daily egg (high nutritional need). Also the needs of a chicken on pasture is very different than a chicken raised in a pen- the chickens who roam on acreage with actual grass and bugs to eat, probably do not need commercial food on a daily basis, or at least not as much of it! And last but not least- breed makes a difference IMO. CA whites or leghorns should probably have layer ration all the time if they want it- they may lay an egg a day year round. A big lumbering cochin who is broody half the time and when laying only lays a small egg- she can probably do fine on less 'ideal' rations. My gals have layer food out all the time, but usually only eat it right when they get up and right when they go to bed- the rest of the time they are out in the field.