Omnistone isn't completely RNG, they have a "smart" selection process which increases the chance of you getting a favourable rune according to the situation:

Conqueror when the user and other enemy champions have high health, further increased chance for Melee champion.
Fleet Footwork if the user is low health and below level 9.
Dark Harvest when near low health enemy champions.
Hail of Blades when near low health enemy champions and building attack speed or attack damage.
Predator when not near enemy champions, and less likely to get it when in combat with nearby enemy champions.
Summon Aery if the user has Heal power icon.png healing and shielding power and are near a low health ally who is in combat.

That's from the wiki. You're right that it's RNG, but not complete RNG. I think it can be good in certain matchups since Camille can use every rune decently. Some guy got to masters playing omnistone Sion only.
Your point still stands though, it's hard to use, especially in solo queue, but don't you think it's possible to use it if you get good enough at it? Plus I love omnistone so I have some bias.