It's rare because B6 doesn't interline other than with EK and the likes. None of the domestic carriers. This means that if you are rerouted on another carrier, B6 is buying a ticket for you at the walk-up price.

That's why the exceptions to the general rule are few and far between.

Don't plan on it and have a backup plan of your own ready. If your B6 flight is significantly delayed or cancelled you are always entitled to cancel for a full refund, so you do have some cash to play with.