The thing with the MBand MBP is that there is unlikely to be anything to demonstrate. Yes, they may well be pretty, but that's not really enough for a presentation - the message will get across perfectly well, if not better, with high quality website/press release images. What would happen at the demo - "Here's the new MB and MBP - see how pretty they are! They are still made the same way as before because that lasers and solid metal business is, frankly, prepostorous. Instead, most of the ports are on the power adapter so that its kind of like a docking station. Any questions?". 2 minutes tops?
While the Let's Rock event was pretty light on ground-breaking hardware, it had software, particularly the new Genius feature, which really needed SJ to show it off and convince everyone it was a big deal. It wouldn't have worked to have released iTunes 8 without a demo.