Musing on Leggings

Lately the leggings trend has been everywhere, on the streets, in class, at the airport. . . its almost becoming a fashion cliche. But lets face it, we all love it and its probably going to be around for another season or so. What I'm wondering though is this: Do you differentiate them between leggings for under skirts/dresses and very tight black pants for under long shirts? It might sound a little trivial, but I've been wondering this because I always thought that they were tailored differently for different purposes. Tight black pants are typically tailored thicker because they are pants and leggings are more sheer and super tight for skirts, right? At least, this is how mine are made. So for under shirts where your butt is half exposed as in Mary Kate here (left), do you wear black pants? I mean, no one needs to be that acquainted with your butt. And for under long shirts where its not as in Ashley here (right), then don't you wear leggings/tights? (ps. don't they both look fab here with their cute short fur coats? I always thought fur coats were for older ladies, but I stand mistaken)
Ditto for Lindsey here. I wonder if they are tights or pants? I'm leaning towards pants because of how dark it looks and the crinkle at her knees.

Thoughts on this issue? How do YOU wear the trend?