That's because many engines come out of the manufacturer or maintenance shop with poorly set up fuel/induction systems. Staying ROP helps to cover up the poor quality.
The curves Jabba has posted many times shows why: The slope of the power curve LOP is steeper than ROP, and therefore differences in the power output of each cylinder as a consequence of the imbalances in F/A in to each cylinder become more evident LOP. "Lean misfire" and "roughness" while leaning is actually vibration caused by the imbalance in the power outputs of different cylinders.
The term "conforming engine" is shorthand for one that's been set up properly. That said, my understanding is that some carbureted engines are notoriously difficult to run smoothly ROP, because there isn't much that can adjusted to deal with imbalance problem.
Some people are happy to throw fuel at the problem caused by poor manufacture or maintenance. That's their decision. The main safety message they need to understand is the need to ensure all cylinders are operating sufficiently ROP. That's because the setting that puts the cylinders under the most pressure and temperature stress is, in fact, a setting ROP: around 50 degrees F ROP.
That's also why an engine monitor is important, even if you don't want to operate LOP.