“Tap isn’t being used as a dance language here, much less as music. It’s being used as a sparkly outfit, and as a symbol of Broadway’s past. These numbers are fantasies. Two of the new ones are hallucinations.” Brian Seibert on the incongruous tap routines that have been turning up in new and recent shows from Mean Girls to SpongeBob SquarePants to Escape to Margaritaville and back to The Book of Mormon. (Seibert doesn’t entirely approve.)