I really enjoyed this weeks episode. I found it exciting but also it created a sense of anticipation for me. For example, when Darkrai was defeated (which was a shock for most people if not everyone) thoughts went thought my mind that I didnt think would ever appear. In this case I thought he was going to pull out another Darkrai to finish off the battle but no, a latios sprung out and impressed me even more. I think the writers used these legendaries because they were unique to a league for a start so it was something different but also legendaries are seen as the eye catchers of pokemon so when little kids see that on TV they go 'I want that pokemon I must go to the latest event to get one or trade with someone immeadiately!'
There is also one more reason which concerns me a little. I get the sense that the writers are scared that if Ash wins a league that is the end of his journey and thats no more show that includes Ash and they risk introducing a new main character which in my opinion I wouldn't want. I don't think it would be the end of Ash's story even if he wins a league. I think a pokemon master should be able to win leagues in numerous regions so Ash can still move on to another region to compete in their league and it doesnt make much of a difference because he uses an entirely new team. The only time Ash needs to go to the elite four of champions league is when the pokemon animé is coming to an end and it finishes in a huge finale.