In the Evolt.org article Dynamic Elements Chris Heilmann explains why hiding and showing elements as a result of user interaction can reduce both usability and accessibility.
This is something I have argued many times over the years. Certain types of designers and clients are very fond of adding “dynamic” elements to web pages. Sometimes they admit it’s because they think it’s “cool”, but most of the time they try the “increased usability” or “three click” arguments. Chris’ article explains the accessibility problems that are common in designs that hide and show elements.