Well, I thought the same thing. I initially bought the ones with the ankle strap, these,
http://www.snaz75.com/el-m-brook.html, but I returned them. They were uncomfortable and I didn't like the "line" it created at your ankle. Also, they just didn't feel right to me. The slide on ones were much easier to put on and off backstage too, because I didn't want to walk around in those things when I didn't really have to. The girls with the straps were kind of stuck wearing them longer since they couldn't easily slide them on.
Just little tidbits.