The writers were just starting out, and they needed to figure out a way to feature every single one of the original 151 pokemon before their 80-episode-1-movie deal was up. They knew that Ash would use the four biggest cash-cows (Pikachu, Bulbasaur, Squirtle and the Charmander line) as his main team, and that left a hell of a lot of pokemon to find a feature episode for. They apparently thought that introducing a species as a temporary team-mate was completely legit and could provide some nice moments, but those characters would only serve their purpose for a short time and simply be part of the journey. That was the logic they went into the show with. Each of the temporary captures had their purpose;
-Caterpie, who was noticeably alone rather than a member of a swarm, wanted to fly, and Ash raised it to be a Butterfree and achieve that dream. It then brought things full circle by wanting to fly away and have its own Caterpie babies and raise a family. Butterfree was never meant to be a battling pokemon, it was meant to showcase the Caterpie evolution line and be developed as a character on their own little journey.
-Haunter was just part of the showcase for Ghost-types (which tied into the debut of Abra/Kadabra) and to be comic relief. Gastly had a major episode, and Gengar would have a large (literally) role in a later one, so this was Haunter's extra bit of a featured role. That's all it was ever going to be.
-And then we come to Primeape, who primarily showcased the Mankey/Primeape family in its debut episode, then was used for a significant challenge and a little character development for Ash, and its departure episode was a chance to showcase all those other Fighting-types which they needed to get through. Primeape did everything it was ever intended to do.
