If you run your own hazards and screens, you surely won't like Defog...
I generally like sweeping a playing field nice and clean of hazards before I set my own, actually. Defog's just peachy for that, especially on a Pokemon that can swat hazard setters.
Bonus: The Evasion Down means slapping something silly with Hydro Pump or the like is all the sweeter. Empoleon loves it.