If a double-faced card is exiled this way, it will return to the battlefield with its front face up, regardless of which face was up when it was exiled
The ignored roll is considered never to have happened
The inability to block is going to matter in damage races, so Bloodfeather Phoenix will be worse the more likely you are to play an aggressive mirror in a Cube, but we often see the aggressive decks off on their own islands in the digital Cubes, and in those environments, cards like this are great
Unlike other sources of ramp, like creatures or artifacts that tap for mana, lands are very difficult to remove from the battlefield, and unlike temporary ramp, like Dark Ritual, extra lands continue to provide an advantage turn after turn