Because reading is not just a utilitarian activity. Reading widely—things that may not be immediately useful, things that may be against your belief, things that may just turn out to be completely wrong—is an important step towards forming a critical appreciation of anything you may encounter.
Personally, I've long come to realize that much of my attitude towards everything from work to politics has been shaped by reading materials that often covered wholly unrelated topics. More importantly (and much to my chagrin), it seems that many crucial lessons came from works that I outright hated and was forced to drudge through against my will.
I don't mean to discount your conclusion: Almost always, almost everybody doesn't know what they're talking about. But almost always, almost everybody is a little right, and bits and pieces you pick up from the most unusual places will inform your solution to a problem many years down the road, or at least remind you that every story has more than one side.