Without Fire Blast, Rain Volcarona is forced to use Substitute. Otherwise, it threatens to be T-Waved / Leeched by daring Ferrothorns that decides to stay in. It's counter-productive, if Volcarona cannot set up on the one thing it's supposed to set up. Changing the subject to Volcarona on non-weather teams - what do you think of Will-O-Wisp? With Wisp, Volcarona becomes more than just a late-game sweeper, but it can also support the team by luring in physical Sweepers like Terrakion, Dragonite, and Gyarados, and burn them. Burning Water-types like Politoed, Jellicent, and Gastrodon would make it even quicker to checkmate them with a Volcarona sweep later on. I guess it depends on how you play, but I like to inject Volcarona early and mid-game to force plays on the opponent, so being able to burn Pokemon would potentially make Volcarona more useful. Heatran is a problem, but all Volcarona without HP Ground are weak to it anyways. Volcarona's access to Giga Drain somewhat makes Wisp half as appealing, since the former can heavily damage Water-types, but Wisp is nonetheless a good option against any non-Fire types.