So lets bottom line it:
If you don't grease, your ball and coupler will wear out faster
If you are relying on the hitch ball for your electrical ground, you are making a mistake in the first place.
You run a better chance of loosening your hitch ball if you run it dry and squeaky.
Downside to the greasing is the mess potential and dirt attraction. Both can be mitigated by cleaning and covering with a cover. And then their is SLOTH. Lots of people are slothful and don't want to do any more maintenance than they have to.
They then attempt to justify their sloth by using excuses that it might get their wife's dress dirty. Why yes it will if you con't clean and cover.
One of the best time worn excuses is "I don't have time" Bravo Sierra. If you have time to post on TBN, you have time for almost anything.
So Guys, get out there and grease those balls.