I think that these "gymnastics" or acrobatic techniques are starting to become a part of tkd. Take a look at this video:
I think ktigers are among some of the best martial arts, especially for poomse. That being said they call these acrobatics moves show tae kwon. An art specifically developed for demonstrations. I don't think there is a line between taekwondo and gymnastics. Taekwondo, while ancient is still a somewhat evolving art. While it is true that you may never use a 720 or 540 in a fight, this fact definitely does not diminish the skill necessary to perform these kicks.