I am more of a Misty fan, but people have got to face facts. May actually had a specific purpose of her own to fulfill on her journey whilst still traveling with Ash and co. Misty was just like, sorry to say, eye candy for the audience, or Ash's main cheering squad. Most people like Misty for her joy to water pokemon, or her temper towards stupidy as in psyduck or dumb comments made from stupid people. But other than that, she could not become a better gym leader or water pokemon master by being in Ash's personal cheering section. It would make more sense to bring May back for she'll actually have something to do.