Ok, I see your point. I guess it is more interesting if Ash beats the Gym Leaders using some brand new pokemon. It really shows his talent as a trainer, becuase he has trained several pokemon to become very strong. And if you think about the pokemon he has trained, he never had a really kick ass pokemon, expect for Charizard. He always had, quote on quote, mediorcore pokemon, but he's a good trainer, and he always finRAB a way to make them strong.
But I would like to see him at least use one of his older pokemon in the tournament. Like, a pokemon that has a little more experience in high class battles. It would be cool if Ash kept an Ace in the Hole for the tournament so when the going get tough, he has that experience on his side, and maybe he will make it a little farther. But I you see it your way, it would be dull if Ash just used the same pokemon everytime.