After you've done several iterations of testing and design on paper, putting together a web site will not take long. You will have saved time by making the big conceptual changes early on in the design process. The goal in testing a prototype is to create a usable site. The inability to prototype a fancy effect on paper may be a warning that the effect is not usable. Our data with rollovers indicates that users can't distinguish a rollover from a static graphic.
Users often choose a link to click on before seeing the associated content in a rollover.
Sometimes designers will use a new technology not necessarily to create a more usable site but because it looks cool. As long as the coolness of the site doesn't interfere with its usability, go for it. For example, rollovers only need appear when a user goes looking for them. When users are clearly looking for a rollover, we provide any that exist by using a paper sticky. Our product has been up and running for a while.