It depends on the enemies intelligence. An intelligent enemy should know fog cloud would be effective against a cluster of ranged attackers.
Fog Cloud definitely has value in any situation there is a cluster of ranged attackers. It doesn't need high ground Adv/Dis for this to be effective. I don't think the player should be over-incentivized to make good choices.
In the world we get a +2/-2 Fog Cloud still has the additional incentive.
I was thinking in terms of using fog cloud on yourself to hide in while closing the distance to the attackers or fighting in it while obscuring yourself from the attackers. If you are in a Fog Cloud you cannot be seen by ranged attackers so they cannot shoot you. Same with Darkness.