Yup, it's on Sigma's HSM motors, too. For many years, it was only available on the EOS system, and then only with those lenses with "proper" USMs, not the ring-type (the only ring type USM lens with it is the 50mm f1.4). Do a search in any Canon forum that's been running for any length of time and you'll find loads of posts relating to "FTM", or Full Time Manual focus. It IS sometimes useful, but it's certainly not critical to my photography, and I would put it in the "nice to have" bracket, personally. It's one of those innovations that took a long time to spread because it depends really on having the AF motors in the lenses, which was not the direction taken by most manufacturers at first, following the enormous success of Minolta's 7000 with the cheaper in-body AF motor.