This is an awesome thread - I love that we can have such a lively discussion about something, disagree with respect, and have fun along the way!
Cardiffgirl, that's my issue with 'leggings' as pants - and maybe it's all about what we call things, as has been noted. If I can see through them, then to me they are tights. Tights are not pants. They are a somewhat more opaque version of nylons. But tights and leggings are also two different things - but the names are used interchangeably, hence some of the issues!
Leggings to me are thick enough to cover what is under them, once that's done, whether you shirt covers your butt is a matter of personal comfort and taste. To Anna's point, once you're covering your undies and following whatever professional guidelines or whatever needs to be met, it's really up to the individual to decide what's right for them!
Gretchen, dance mom high-five - my daughter is in the studio 5 or 6 days a week, so I can't really blame her for living in leggings, even though I'd love to see her in something else. But we have a strict "once it's generally appropriate it's none of my business" policy for the kids clothing choices too - after all, she survived going to school all winter with summer shorts over her pants for two years of kindergarten, so leggings aren't going to do any harm now!