I have a family member who talks, without any irony or embarrassment, about how he can't remember anything he reads. Obviously, he's not a writer.
If you don't remember what you've read, how can you be inspired by what you read? How can you capture cross-fertilizations among the things you've read?
Wouldn't it be a terrible shame if the best idea you'd have had this year came from a synergy between a book you read two years ago and a Wall Street Journal article you read this morning? But you completely forgot about the book, so that synergy--and the amazing ideas that would have come from it--never occurred.
Think of all of the hours you wasted reading books, magazines, newspapers and blog posts when you forget what you read. Isn't life too precious for that?