In fairness, wasn't Deathly Hallows (the book) released around the same time of the film so they couldn't really have known how significant the mirror would be.It's a perfect example, actually, of something left out of the films and later regretted!
At one point (but maybe not anymore?) they were consulting with Rowling so they would know what bits were okay to excise. Maybe they came up with a workaround that she approved.
From what I hear, she was notoriously secretive with her work and wouldn't even let them in on the slightest secrets, even though the films could have ultimately benefited from it. If she knew that she was going to hold the littlest details from the film makers, she should have told them to wait it out. Hell, I think the movies would have made even more money if they were the only Harry Potter stuff being released.