The reason that Doctor's Orders is good for catching Jimmy Jenkins is that normally, you have an extremely small chance of spawning any loot loader/midget, and within that small chance, Jimmy Jenkins is the rarest of the potential loaders that can spawn. While Doctor's Orders is active, there are 4 cardboard boxes at the Wildlife Preserve that are guaranteed to spawn a loader. You still only have a small chance of one of those loaders being Jimmy, but it's much faster to farm a Jimmy when you have a fast and guaranteed source of loaders in the first place, versus farming for hours and potentially not seeing any loaders at all, much less a Jimmy.