TKD can be very usefull, especially if you can kick like Bill Superfoot Wallace or GM He Il Cho!
TKD is mostly kicking so if you are in a position to fight someone at kicking range, you've got an advantage. Accomplished black belts in TKD can kick very hard, very fast at about any target. They don't throw low kicks as hard as thai boxers, but they've got a side kick which they can throw as bout as fast as a boxer's jab with 3x the power.
A spinning back kick will level someone if it lands in the center of mass.
TKD lacks infighting and grappling though. Some TKD schools teach HKD or judo, which is a good suppliment.
Last but not least, real TKD fighters (and not kids trying to show-off) perfect about 3-4 kicks MAX! They don't waster their time working on 360 aerial spin kicks; they probably perfect their front leg round house, side and hook kick and maybe a spinning back kick or some variation thereof.
Its mostly the teens who try to show off by doing aerial 360 kicks and tornado style kicks. Some whiz-kids can use them in sparring, but if I'm going to spar in a non-boxing stand up format, I want to be able to throw leg kicks
