Heh, I am still enjoying this thread.
It occurs to me that complaining about leggings being see-through is a bit like saying you don't wear t-shirts because they're sheer. It's an overgeneralization. Many t-shirts are thin enough to show off more than most of us want to, but there are plenty of substantial t-shirts out there too.
Of course, there are many folks who don't wear t-shirts as standalone tops, or only wear them as gear (see the pattern?). Weren't t-shirts sort of the leggings on the 20th century? Undergarments that migrated into casual wear?