It's probably good exercise - most of the dances are quite fast. Like any sort of extra-curricular activity it is up to you and your child how far you take it. Lots of girls try dance classes of various sorts (eg ballet) and give it up after a couple of years.
I totally agree about the wigs btw but think they are a fairly recent thing. Don't remember them when I was young but I didn't exactly make it to Riverdance!