In that case, Grease actually works against me more than it works for me. If a creature doesn't move, I get no benefit. If they do and they fall, it's harder for me to hit them. There's only a tiny space for benefit, which is if I'm engaged in melee with the creature in question, and that's somewhere I don't want to be unless I know I can either get good damage off on them or I have little other choice.
@bears: Again, prone is terrible for me. Being prone doesn't deny a foe their dex bonus, so no sneak attack, plus they get a +4 to ac vs. ranged attacks. I'd be wasting an action if I attacked the prone dog.