There has to be constraint to the rock or you run into the problem of it being too powerful. Too powerful means the story has nowhere to go. Conflict can be eliminated immediately and permanently. Ultimate power makes for a very short and boring story.
Take Superman for instance. He has limits on his strength, speed, vision powers. He has weaknesses to kryptonite, red sunlight, and magic. One might even call his unflagging moral compass a weakness. And even then the character is hard to handle and write for. There's a reason that 4 of the top 5 rated Superman stories either don't have him in them, are alternate realities, or focus on the Clark Kent side of the character. It's because conflict with what amounts to a god, is very... challenging.
When John Byrne took over the character in the 1980s, the first thing he did was de-power him from the planet-moving character that he was in the silver age. He could literally move planets around like marbles. You don't have much room to work with that sort of power level. Byrne made him a lot more human. And in doing so, he made him more relatable to the average reader. The human brain is a lot more able to wrap itself around the idea of a guy that can toss a tank a really long distance as opposed to one that is able to destroy planets.
Jon, and his little rock, have constraints. SMALL ones as far as I'm concerned, but constraints none-the-less. And in those constraints, the writers can move around and create conflict, human interaction that the reader can relate to, and build on towards future stories.
The reason that the threads where major loopholes are found and exploited don't go anywhere... is because they have nowhere to go. Whoever has the rock at that point becomes omnipotent and omnipotence is ultimately boring.
