This is such a big topic and so important. We are what we eat, and as much as possible I like to know what has gone into what I eat.
I have heard over and over that prepared feeds are very well balanced and birds don't really need anything else in the way of feed. I think from a cost/benefit point of view they are probably right, I also believe they are indeed a well balanced diet. What bothers me though, is all the chemicals, and other stuff in there that is not on the label, and if you knew what is was you may choose not to feed it to your chickens. That is just my opinion though, as I don't eat organ meat, that certainly doesn't mean it is not tasty and healthy it just isn't to my liking.
I do use some pullet grower, and some layer ration and I do start my chicks on medicated chick starter. But For the most part(over 90%), I try and give my birds things I have put together myself. They certainly seem to prefer almost anything over pullet grower given a choice. They seem to hate layer ration.
I think what people feed their birds by necessity depends on factors like how much space they have to store large feed bags, and how many chickens they are feeding, as if you only have a dozen birds it takes to long for them to go through all those bags of feed and it can go bad. It also depends, on what you can source locally, or off your own land as that impacts price so radically.
Protein is really important and if you can't free range them, if you don't have access to whole milk, or farm raised meat then providing protein can be a real problem and pre mixed feeds do have the necessary protein just not from a source most of would feel completely comfortable with.
If you are selling meat or eggs then cost is really important. If you are doing it for fun and your own family and friends than cost is not so much a factor.
I don't think there is a one size fits all way of feeding birds, and the degree to which you mix your feed varies from person to person.