It really is.
I used to think it was frustrating when I was a small child and it took me a long time to finish a book or have it read to me. But now I think it's a good rule, and I'll definitely adhere to it if I have children.
With LotR, it's especially important. If i hadn't read the books before I saw the movie, then I wouldn't have had a clue what was going on. xD I'm blond enough as it is when it comes to understanding what's going on in a movie, so I don't even want to think about how confusing it would have been if I hadn't already known the storyline.