It seems most people are in agreement on the opinion, if not fact, Omnistone is niche at best and absolutely worthless at worst. And its existence does seem like it was forced into the game after Riot FINALLY conceded that they could not properly balance Kkeptomancy,
Omnistone isn't even its own keystpme. It's just a randomly generated keystone from existing choices and switches to whatever the game deems is the right rune for the job. And just so the Inspiration tree can have 3 choices for keystones.
But does it really need 3 just to round out the numbers? It seems like a lazy keystone backed by a lazy excuse.