A major reason for giving live talks is that you can react to the audience and adjust your talk. This is difficult with slides. Even experienced speakers overestimate how much information they can convey in a lecture and are stuck with flashing through slides that almost no one is understanding. I have heard good talks using slides, but they are rare.
On some anniversary BBC.com asked readers for their view on PowerPoint. According to the responses no one had ever heard a good PowerPoint talk.
My advice is to never use slides/beamer when a blackboard available.
Paul Halmos once wrote: even a good expository paper, read out loud, would make an awful lecture --- but no worse than some I have heard.
Too many beamer talks are expository papers read out loud.
