Regarding balanced diets, treats, and varied diets....As a veteran rabbit breeder (30 years), I'm pretty darn well versed in dietary requirements and how to mess them up.
If you are feeding a complete-balanced-but-seemingly-monotonous diet, you *can* spice it up a little by providing a WIDE variety of additional food items in reasonable amounts. A varied diet can indeed be balanced and complete, but it takes a fair bit of research to ensure that what you are giving does not wind up unbalanced or incomplete.
To the dismay of those in the 'the feed companies are jerks' camp,

, these folks do indeed know what they're doing. Their jobs depend on providing good, solid diets that do what they are supposed to do, and do it well. Yes, there are feed mills out there who do a lousy job.

But there are also mills which pride themselves on doing the best possible job! I currently feed my laying flock (banty orpingtons, a buff LF orpington, SL Wyandotte and two easter eggers) Nutrena All Flock and am totally satisfied with the health, weight, and production of my flock, for example. But they, too, get treat stuff here and there to break up the monotony--sometimes scratch, sometimes mealies, sometimes a clump of fresh grass or bunch of weeds. But always in moderation!
None of this means that treats or mealies are a Bad Thing, though....it just means that we each should do our research on just how much of various treats and supplements are appropriate and reasonable.
Many feeds, by themselves, lack various nutrients or have one or more in serious excess; it's good to know which those are before you just start tossing feeds in the pen for your flock.`