Agreed 100%. I have tried the marketing first, demo second approach and it doesn't work. Either you run out of time or you lose the audience if you start off with marketing. If I want to know the marketing stuff, I can get it from your blog and website or as a link AFTER the demo.
Show, don't tell. I've been on the receiving end of a great many WebEx-style demos over the years. The usual pattern is slides first, explaining the company, its market, and its product. Then, if there's time, comes the demo.
Though it may seem counterintuitive, I'd rather do it the other way around. For me, and for the viewer, the ideal screencast begins with a very brief stage-setting introduction. But it's mainly a compelling demonstration of, and conversation about, a product in actual use. There are few, if any, slides.