Naw, not really true. There's a lot to imagine with a graphic novel. Ever hear "A picture is worth a thousand words"? Someone could write three pages describing some worn out, webbed corner room, and some people could draw it out. Either way, it gets appreciated. Not to mention, there's a lot of context with any story, whether visual or written. I mean, my imagination goes wild either way. In Harry Potter, I always wonder more about Voldemort and more into his education of Hogwarts. Did anything more happen that was never written on paper? Same goes with the graphic novel Strain, what happened to this guy to make him so conflicted when all it says is his childhood was troubled?
A lot of the information we receive daily is visual, excluding blind people, so it's more than just a "picture book". There's a legitimate story in graphic novels, there's a story behind almost anything when you think about it.
Take this for example:
This is a picture taken on September 11 this year. It's Robert Peraza, who lost his son during 9/11 and took a moment to pause at his name on the memorial. It's a beautiful picture and it captures a lot of emotion, even if you didn't know anything about why this man was kneeling at a plaque. A good graphic novel, you probably don't need to read to get a good grasp of the story in it, but then again, that's what art sorta does.
Anyways, to each his own. Some people understand better when they read, and some people, like me, understand better visually.