Well, I admit that when there is an anime based off a manga, I often watch the anime by choice rather than to read the manga. Even when people say the manga is longer or better. There are also exceptions when I've read/watched both and compared. Manga is often better!
But with pokémon, which I'm not sure everybody realizes, there isn't just ONE official manga that the anime is based on. The anime is completely standalone and there are MANY manga with different stories, characters and worlds. None of which have been turned into an anime so far, from what I know. Rather, one of the Kanto manga and some from other regions are more or less loosely based on the events of Ash's anime.
That is why so many wish for an anime based on Pokémon Adventures. It could be made more mature and very different from the current anime, since the characters and plots are so different.