Then the following episode is a water race where Ash decides to use... Pikachu! Can someone tell me how using Buizel, Oshawott, etc. wouldn't be more natural than that? I remember that back in the day, if there was a Tauros competition, Ash would get his Tauros for an episode, if there was a sumo competition, Ash would get Snorlax, if there was a water type tournament, Ash would get Kingler, if May's Bulbasaur had problems, Ash would ask his very own Bulbasaur for help. That was NATURAL and not forced for the sake of fanservice.
And I don't even want to read the "Ash only uses his reserves at the end of the series" when all these examples are from the middle of a series. And I could also say "Ash always gets at least one new starter" and we all know what happened. Trends can be broken.