Well, yeah, Yngwie used to disconnect the middle pickup, but he doesn't anymore. He still disconnects the tone knobs though. But I've heard both him and Greg Howe explain in interviews how that middle pickup gets in the way of their picking. Of course that doesn't mean they can't play with pickups that aren't lowered :)

Check out this video, at 2:19 Greg Howe explains it all. That's why he doesn't have a middle pickup at all on his new guitars.