Dont forget, there's many FATE bosses that cast spells with a timed cast, not just simple auto attacks and instant WS spam. Slow makes them cast slower. DR reset after 10 or 15 seconds (closer to 15 is what it feels like), so essentially the boss is slowed 20% for 2/3 of the time, pretty useful. They deal less damage to you, your choco spend less time healing and more time attacking.

Just an example, at 50, in crap gear, my WHM's Aero ticks almost twice as hard as the Choco's DoT, so it really doesnt scale well. 20% slow always scale. BLM has a 20% slow, its on a 30s cooldown, its worse than the Choco's and its a class ability.