Originally Posted by Fat Melee Mewtwo
That and there are a lot of powerful offensive threats that have no problem switching into Celebii. Perish Song isn't going to matter for powerhouses like Salamence or Heracross only need one turn to wreck havoc. A weak U-Turn won't really cut it that much as Celebii can't afford to be spamming it all the time anymore than it can risk giving a free switch-in by using one of its other moves. It counters Team 1's picks but isn't hard at all to counter. Like almost any defensive mon, it's not putting any pressure on the opposing team's future picks.
The reason why I suggested ThunderWave over Perish Song. It will cripple sweepers Perish Song can't handle. U-turn really isn't to deal damage, it is to gain momentum. Overall, Celebi can handle Rotom-W and Landorus, Mons that can really cause us problems.