Yes, it's a great nature, even for competitive play (not as much as Timid or Modest obviously) but definitely for a casual playthrough of White. Even if you intend to EV train it and make it as competitive as possible, it's still one of the best natures. This is because most if not all your in-game opponents have poor IVs and are not EV trained.
Further, Volcrarona's base 100 Speed already makes it much faster than most of the opposition anyway. And then there's the issue of its Defense stat, which is its worst at 65, and most of the Flying or Rock attacks you'll face are Physical, so you're better off switching than staying in. Since its Defense is so low, it almost makes no difference if you have a Defense reducing nature (you can even use an X Defense to buff it up, if that's your playstyle) because
a) it's fast enough to outspeed and strong enough to OHKO
b) it's always better to switch out of a potential super effective move than stay in to face it.
c) the case where the drop in defense makes a non-KO a KO is very difficult to predict, which is to say a lot of the times if even a defense buff can't save Volcarona then a defense drop doesn't matter because for an in-game playthrough, the focus is on offense for Pokemon like Volcarona, so it's really not worth thinking about these rare fringe cases in my opinion.
Finally let's take a look at the strongest trainers in the game: Alder, Cynthia, and Morimoto.
For Alder, the only problem is Bouffalant's Stone Edge, but even a STAB Head Charge is bad news for Volcarona, so you're better off switching. None of Accelgor, Druddigon, Vanilluxe, Escavalier, or Volcarona give you much trouble, although it's better to use other Pokemon to deal with Druddigon and Volcarona.
In Cynthia's case, Braviary's Brave Bird and Garchomp's Stone Edge are the biggest hurdles, followed by Milotic. If other Pokemon can deal with these, then you won't much issue.
For Morimoto, you run into a bit of trouble because Simisage, Simisear, and Simipour all have Acrobatics and his Swoobat has Air Slash. So only in this case, would having a low defense somewhat make Volcarona take the bench for this fight, but that would be the case even without the lowering nature.
