Ideally we don't want to see maneuvers on a barrel that require a 'come, switch' command. It just doesn't work well in the real world, and even the advanced dogs who have been raised on barrels don't handle it well enough for us to want to see it. They can do it, but it's not what it should be for a dog who was raised with that obstacle and should be proficient with it.
Now, that isn't to say you'll never see it. Sometimes a course designer gets stuck and you don't want to redesign an entire course. So it happens. But we don't want to see anyone intentionally putting that sequence on courses.