Originally Posted by Kenzor
Pretty sure anime is made under the assumption that 100% of viewers in Japan have already read/watched the original material hence why anime openings usually contain spoilers.
I don't know why you assume that. In the western market, trailers for movies often display the best parts, spoiling the movie and giving away what's going to happen. Plus there's a high chance you don't even know the original source material some of them are based on.
With anime openings, I often find that they don't show a damn thing from the anime, outside of a few second clips (or none at all). It's mostly just a pretty OVA to show off to the fans how nice of an animation budget they have. Same thing with the ending.
Because it's that opening and ending animation that gets most of the special attention in Japan. They're not going to want the more half-assed animation from the actual show in there. People rave over openings so much, that some will actually watch them every single time they start up an episode. With some buying the Blu-ray boxed set just for the opening and ending clips in top quality.
To finish, no one can also assume that everyone reads the manga. There's a reason why there are different media adaptations.
Such as catering to those who may not like reading manga, who simply don't have the time, or the living space to store 70 volumes of this crap. (Assuming there's no manga cafe nearby.)