Carl is right.
Grease can be "pushed away" and not do a lot of good.
However, because I know I'm going to get dirty today doesn't mean I'm not going to take a shower...
There are no containment seals on a fifth wheel plate on a semi, yet they grease them before every trip.
There are no containment seals on the pivot pins on my backhoe, but if we don't grease them every couple days, the pins and bushings wear badly, requiring replacement.
There is a tremendous amount of friction/wear between the ball and fixed forward and top portion of the coupler, and the adjustable "lock" part of the coupler.
Couplers and balls can wear to the point that the lock no longer "locks" and the trailer can come free of the ball.
Most people don't notice wear as there is usually enough tongue weight to keep the trailer and ball together.
There are many trailer manufactures who include greasing the ball in their manuals. I've never found one that says "you shouldn't".
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...