The story of Spec Ops' campaign revolves around the fact that a giant sandstorm has separated Dubai from the rest of the world, and there are sequences where the player fights their way through one, but those are pre-designed battles. The sandstorms in multiplayer can come at any time, so there's no telling how it will change the situation on the battlefield.
Map fatigue is concern in all multiplayer games, but Spec Ops has a wrench that can keep games interesting for players even after after cycling through the game's maps a few times. A sandstorm can blow in on any map at any time, leaving your squad without communication, and players unable to see beyond what's directly in front of them.
Sandstorms are nothing to scoff at, players on ziplines or narrow platforms can be blown away, sending them to their deaths. More commonly, when players have such low visibility it's very easy to run into an enemy and not realize it until it's too late.