I believe that a PowerPoint or some form of slideshow is almost always beneficial for the retention of a speech. It doesn't have to be an intricate slideshow or very detailed at all, but something with a few main points to help the audience make a visual connection to the information will always create better retention. Of course there will always be some distraction with a slideshow, whether it be because of a typo or an interesting photo on a slide that distracts the audience, however I think the overall benefits far outweigh the negatives of using a slideshow in a speech.
I would agree that a slideshow will always be a better fallback then not incorporating one. I personally have always benefitted greater from visual aid rather than just auditory.