For something you want to capture immediately, the amount of time it takes for the phone's accelerometer to decide you have rotated it is already too long.
If, and only if, the application supports it. Frustratingly, not all do, so you're stuck with the biggest black bars framing a microscopic landscape video.
Contrast with a monitor, where it will at least be viewable vertically, even if it too only fills a portion of the monitor horizontally.