Yeah but the Zanpaktou's reflect the user's personality somewhat don't they? You wouldn't imagine Kenpachi with Rukia's dancing shikai now would you...
It would explain why he doesn't use it, now, wouldn't it?
Also, you gotta wonder, when Kubo was writing this one, was he sitting there thinking:
oh boy, they'll never see this coming, they thought his ability was something else. Man, am I a good writer or what?