Krisz

As Norbi said above, because of legendary pokémons are really rare even in anime and in other games, I don't think that it is a great idea to catch em in the wild. Though, being able to catch em by winning some tournament is just an awesome idea! It is a perfect prize for it, it makes many people to join the tournament, and everyone can feel the difficulty to catch one. Also making legendaries not tradeable is a 'must', so no one can make money from em, and so on.