^above
The point is that Rotom-C will want to stop the Volt-turn chain, so if your own Rotom-C isn't scarfed, every time you switch in against Rotom-W's VS, the opponent will bring in Landorus, forcing you out if you don't want to lose 70,25%-83,06% from EB Landorus's U-turn or 58,68%-69,42% from Scarf/lefties Landorus (assuming 4 HP Rotom-C). But if you don't run Speed you can underspeed Celebi and VS after him, so you are right about Celebi...
The point is that Rotom-C will want to stop the Volt-turn chain, so if your own Rotom-C isn't scarfed, every time you switch in against Rotom-W's VS, the opponent will bring in Landorus, forcing you out if you don't want to lose 70,25%-83,06% from EB Landorus's U-turn or 58,68%-69,42% from Scarf/lefties Landorus (assuming 4 HP Rotom-C). But if you don't run Speed you can underspeed Celebi and VS after him, so you are right about Celebi...