That's the problem. Phione is average all around, making it subpar to other options you may have. The only merit Phione has is abusing its ability, Hydration, by setting up rain and spamming Scald or Toxic to spread status throughout your opponent's team, and then use Rest and wake up immediately after. Thus, Phione can provide decent team support.
However, it's not stellar. At setting up weather, Politoed is probably a better option as a moveset isn't wasted on it completely. However, if you are playing PU / NU, Prankster users have priority rain, and Phione doesn't have the best of bulk or speed to back it up.
So if you want to abuse it's Hydration, go right ahead and do so. Otherwise, you have better options to consider.