One of the things I learned about memory when I was reading tons of papers on this stuff is that usually people need to have comprehension of something before they will remember it. As an example, when I was teaching English to high school students in Japan one time I told the students to bring their textbook the next class (we usually didn't use it). I spoke English. The next day, not one of the 30 or students brought their text book. I asked the students, "Does anyone remember me saying something last class?" Not one student remembered. I had a teacher teaching with me ("team teaching" -- very good idea) and he remembered me saying it. It was then that I really grasped the concept: if you don't understand, you won't remember. In fact, you might not even remember that something you didn't understand had occurred.

I think what visualisation (especially with story telling) does is create a framework for comprehension. It gives logic to the thing you are memorising. This is especially good when you are memorising a list of arbitrary facts that have no inner logic themselves. People who are studying Chinese characters often create stories (ala Hesig) and I can verify that it makes them easier to remember. Without an inner logic, it's really hard to hang on to.

My feeling (which corresponds to my experience) is that when memorising things that already have an inner logic, there is no need to visualise or create stories. For example, if you memorise example sentences rather than vocabulary or grammar, visualisation doesn't help. The logic already exists within the sentence.