The majority of slides in an average presentation are the bubble wrap that protects the real content. These slides are summary pages, set up pages, and lists of bullet points to remind the speaker what she should be saying next.
One indication that a slide is really needed is that you often look for them in one on one meetings. “Hey, wait, where is that email, with that attachment, with that special slide that I made a while ago.”
Spend more energy making slides like these.