I don't mean to be disrespectful, I totally agree to the consensus, but visualizing things IMHO is detrimental to the point of meditation - being mindful, as joe very well put it - instead of observing, you'd be creating images and assign labels. Just my point of view.

Now, the way I see it, there's a fine line between creating imagery (or daydreaming) and observing a thought in the form of an image, and I believe that you were referring to the latter rather than the former? I just want to get my stuff straight, maybe my logic isn't the best at times, so please correct me if my train of thought is flawed.