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isnt it better to learn and use both in order to not be limited to one or the other?
i mean say u wanna draw somethign beautiful and detailed would u prefer to use realism or manga?
and if u were trying to draw something actiony like a fight between warriors would u use realism or anime?
and that kid who was rejected by his artschool or w/e it was since the school only teaches realism its useless for an anime artist to go there

I would have to say that, if you are at all serious then it is a MUST.

My reasons:

Nothing is more annoying that seeing well done coloring, line art, and the anatomy is greatly off (Beyond even style). It is easy to combat a problem like this, by studying anatomy. If a person takes time to look and practice everything becomes easier.

Another thing is more unique poses. I started with the really cartoony style. I ended up at a stand still for years, and I do mean years. Then at the beginning of this year I listened to a friend and started doing hard core realism studying, and I have improved so much. I can think of unique poses, at the same time take a pose from something else, and motify it to fit what I want.

Another this Realism allows for, is the knowledge of how you, or someone else, is bending anatomy. It is annoying to critique someone, and get a "?" or "it's my style", which happens to change between each picture. If a artist has a style, than it should remain consistent throughout their works. Another use for this is the ability to take any pose (even lines), and change it your style effectively.

The reason a lot of art schools and classes dislike anime people, is that many don't want to draw anything else. All they want to draw is anime. It annoys the professors greatly. (I know this from my own collage).

long story short, if you care realism is the way to go. Plus if you want money, realism tends to for sell more.