I agree, the Aes Sedai are one of the most interesting parts of the book but I think we will need to see them more than we do in EotW. Plus Alanna is one of the Aes Sedai we encounter throughout the series so it makes sense to introduce her from the start.
Maybe they are trying to build up an Aes Sedai presence as it is only really Moiraine and Elaida in EotW. Alanna is a pretty good choice as she is fairly prominent, as she does go to Fal Dara they could also be using her to explain how that arc comes about behind the scenes which we don't get in...