I phrased it wrong. What I tried to say is that you can only use this pokemon to fly, swim and ride. The other pokemons don't have any use other than battling. This legendary going to your team to battle is given.
And the fact that this pokemon is classified as legendary but everybody is riding one on the trailer is just mind-boggling
Having one Pokemon for all the traversal options and it doesn't even take up a slot in your roster (I assume, since it won't be fighting) would have been a nice solution back in the day when I was forced to carry around multiple pokemon just to have Cut/Fly etc, available.
But I suppose people may want to be able to do some of those things using some other pokemon they like.