You see Moonblast more because the majority of Fairies have better Special Attack then Physical Attack. Play Rough is still perfectly viable on physical based Fairy types such as Azumarill.
So for a list that are better with play rough (fully evolved only).
- Azumarill
- Granbull
- Mawile
So, to clarify, Moonblast if a Special based attacker, and Play Rough if a Physical based attacker.