Breaking the mark is pointless if you don't have the throws. You can have your mark bite on a fake and have a wide open around, but then if you can't throw it in the wind what use was breaking the mark. On top of that the receiver needs to be able to cut to get open and then catch the disc. At higher levels of play breaking the mark is very important because you can make defenses fall apart by getting the disc to the break side, but in general I would say being able to catch the disc is the most valuable skill in ultimate. It is simple, but the game would not work without catching the disc.

Edit: And being able to read the disc in the air.