Well I think it all comes down to why you have chickens... If you're keeping them to kill and eat them or solely for their eggs...or if you have a "farmer" mentality, then just feeding a chicken a diet of chicken feed makes sense. You'll have healthy yummy chickens with just chicken feed.
But if you have chickens as pets - just like you'd have a cat or a dog - you want to feed them treats for many reasons - it entertains them, they like them, they'll learn to associate you with something good, and they may even become lap chickens who want to sit with you when you visit them.
So it's probably good to look at why you have chickens when you decide if treats are appropriate.
We want 3 friendly pet chickens and eggs are great, but not the primary reason for owning them. We want them to come when they're called (which they do) and sit with us and just be funny feathered members of our family - so we're doing what we can to entertain them with treats.
Neither way is right or wrong - its just what works for your situation. My dog is our family pet and I wouldn't dream of just feeding him dog food every day. He gets plenty of dog-appropriate treats!