To be truthfully honest, V-Create isn't a great move on Rayquaza. Overheat and Fire Blast hit lots of things hard like V-Create does and have no drops (apart from Overheat). DD sets are sorta redundant with a speed-lowering move, not to mention that 150 SAtk is great.
But without V-create, can Rayquaza, after a Swords Dance, destroy Groudon and Arceus in one hit without locking itself into Outrage for 2-3 turns, and thus rendering itself unable to use ExtremeSpeed during those turns, and therefore leave itself open to being revenge-killed by any faster dragon?
Another benefit of V-create that I can think of lies in the sheer difference in power between this move after a Swords Dance and Overheat. From my experience, many trainers, especially users of stall teams, like to play mindgames against Swords Dance Rayquaza, by switching back and forth between their own Pokémon, trying to wear down Rayquaza with its Life Orb recoil as well as tricking the Sky High Pokémon into hitting their Ferrothorn's Iron Barbs on the switch with Dragon Claw or ExtremeSpeed (as you can observe my opponent attempting to do in
this replay), baiting it into using Overheat by sending out their Ferrothorn, Forretress or Skarmory only to switch to a Pokémon that resists Fire-type moves, and in effect cutting the power of Rayquaza's Overheat in half while making the dragon suffer Life Orb damage in exchange for dealing a pitiful amount of damage on that turn, having their Ferrothorn staying in against Rayquaza to Leech Seed it while predicting Rayquaza to use Dragon Claw instead of Overheat in an attempt to predict a switch, and then stalling it with Protect, and things like that. However, with V-create as Rayquaza's move of choice for dealing with Steel-type Pokémon, I think the Dragon/Flying-type Pokémon would find itself experiencing a much easier time in this situation, as even in the event that the opponent withdraws their Steel-type Pokémon from Rayquaza and sends in something that resists Fire-type moves, even that Pokémon would be unlikely to enjoy taking a +2 V-create on the switch, and would even be pretty much certainly finished off by an ExtremeSpeed on the following turn.
V-create should only be used on choice sets IMO, as the stat drops include speed, and Rayquaza generally needs as much speed as possible.
Considering ExtremeSpeed is Swords Dance Rayquaza's main attack, Rayquaza hardly needs Speed with that moveset, as after using V-create to destroy that which neither Dragon Claw nor ExtremeSpeed can take out in one hit after a Swords Dance, the Sky High Pokémon can then continue sweeping with ExtremeSpeed anyway.