I'm not seeing big appeal to defensive Volcanion at the moment. First of all, it seems to offer some role compression between Heatran and Tentacruel, but it has barely any supportive options compared to either of the two while being slightly less specialized, so it can be difficult to justify the teamslot. What's more, Volcanion is Stealth Rock weak while having no recovery of its own, so it wants further support on that front.
Secondly, it hardly offers any real advantage over offensive Volcanion aside from its use of Toxic and Haze, and I'd hardly believe that offensive Volcanion cannot consider Toxic whatsoever, and I don't really see Haze allowing Volcanion to win any matchups it otherwise could not: most likely Volcanion may just end up getting into stalemates if not still put at a disadvantage against things like Suicune, Clefable, Reuniclus, Mega Altaria, etc. From what I can see, the only real scenario where defensive Volcanion's bulk comes into play is against offensive Mega Scizor, where it tanks the +2 Superpower almost twice as well as offensive Volcanion. Aside from that, defensive Volcanion doesn't really seem to offer quite enough; B+ looks better for it imo.
Also LO Volcanion doesn't really mind the -SpD nature too much since the only special attacks it really wants to be taking are 4x resisted / immune anyway. Plus LO Volcanion retains plenty of holepunching power to be compared to Specs (similarly, CB and Specs Hoopa-U were both S, and Life Orb lingered closely behind in A+).