The pokedex isn't as much for Ash as it is the viewers. A lot of people who aren't familiar with the series might want a bit of briefing before facing a pokemon. It helps tell some of their abilities which aren't easily seen just from how they look. For example, someone new to pokemon might not know that Nidoking is a poison type pokemon, and thus capable of different moves. Plus, a lot of them are really spread out. It's not like he fights Gengar every two episodes.
