I think every Phione should have U-Turn. That's the only thing that makes it unique from other waters. Yeah, Lumineon is the only OTHER water that can learn U-Turn, but it can't really do much.
That leaves you with Phione who is essentially the only water type that can learn U-Turn and use it well. It is incredibly hard to kill if you use a Bulkier Rainresting version with U-Turn and Waterfall, and it actually helps support the team quite a bit by possibly allowing an extra turn for the U-Turn recipient because Phione may be faster than the opponent, therefore not making the switch take a whole turn by in itself, allowing for more rain.
That and U-Turn also helps set it apart from Dewgong. Dewgong doesn't completely outclass Phione, it has many more weaknesses and a stealth rock weak, which Phione does not. Phione can also stay alive much longer thanks to more resistances, and can hit harder thanks to higher attacking stats.
I've used Phione before with moderate success. The big problem, which was already mentioned, it obviously its movepool. Its only good niche is a water type that can learn U-Turn. Use that to your advantage when you're using him.
I don't remember my particular EV spread since I don't use Phione anymore, but I used Waterfall/U-Turn/Rest/Rain Dance.
I think it had enough EVs for 352 HP and then the rest of the EVs distributed mostly into special defense since its weakness (grass/electric) are mostly special, and then enough for about 220 defense.
Yeah, so from my wall of text, basically my message is use U-Turn on all Phione movesets and make it bulky and defensive. :]