A lot of the arguments against wigs, make-up, costumes, etc. in competitive Irish Step Dancing could also be made about competitive figure skating. I have the impression that competitive figure skating does not "mostly concern adults", but involves teenagers and younger children.

Someone mentioned ballroom dancing. I'm sure we could think of any number of sports/activities (e.g. synchronized swimming) that involve judging, and for which anything that the competitors - and more importantly their teachers/handlers - think will give them an edge with the judges is freely done, regardless of expense.

Don