Well for a 17yo to wear that i don't think its a problem, BUT...she has an audience that is aged from about 5 - 13 which she needs to perform for. Having said that she can dress as revealing as she wants when she is going through her day to day life. however when she is performing (be it on stage or music video whatever) she needs to remember that that isn't appropriate for those aged around 5 - 13 (13 borderline) so she just needs to remember that so i think although for her age its not inappropriate and i am fairly sure i have worn more revealing clothes at parties and what have you but as i was saying my audience was not little kids we where as Miley Cyrus's is.
When you are a public figure you need to dress for your audience as you are being watched all the time and anything you do could harm your career. (remember Miley's pole dancing at the awards ceremony when she sung party in the USA
However Optionals rocks has a point!!!! ok i am 15 ok and i wouldn't dance like that like really, well if it was a choreographed routine i wouldn't have a choice but 7,8,9 that is way to outrageous for that age group. Saying that they were very good dancers, the turns were amazing, amazing technique that should be invested in ballet or contemporary, or something of that sort where their brilliant technique will take them places.