Exercising control over muzzle direction is simply forming good habits. I believe that's why everyone else who responded in this thread mentioned practicing good muzzle control even when using firearm simulators. This continues the build the good habit of never pointing anything that feels like a firearm at a person unless you make the conscious choice to do so.
Muzzle control also acts as a signaling device to others. When I see someone using good muzzle control with an unloaded real gun, blue gun, or sim gun, I personally feel much better about being in the vicinity of that person while they are handling a firearm.
As for reloads with the muzzle over the berm, this has nothing to do with the gun being at slide-lock. "Up" is generally considered a safe direction unless you have reason to believe that it isn't. If you go to a USPSA match, you will see nearly every shooter with their muzzles over the berm while doing in-battery reloads.
Muzzle up wouldn't be a ready position if it was considered unsafe to generally use "up" as a safe direction. Personally, I've seen more rounds leave ranges from skipping off the range floor than being directly fired over a berm, which would also preclude low ready if the primary concern was the possibility of a projectile leaving the range due to an unintentional discharge.