The Commodore 64 is a great machine for writing demo's. It has a small memory space and an interesting architecture, making it a great platform for seeing who can present the latest and greatest effect in the most artistic manner, at maximum speed and in minimum space.
If you write a 64 demo for a PAL c64, you know it should run on any other PAL c64 on the planet (ok, unless you rely on open address spaces or try to force DMA on a machine with an older VIC revision...). c64 demos are small and easy to transport over the internet.
The c96 is designed to be a fun alternative demo platform. One possible application would be as an alternative to Java for animated www pages. Just think how much cooler a demo page would be :-)
Of course this presupposes emulators being available for various platforms. Currently there are none.
Plans are afoot for a PowerMac native emulator, and possibly a Java implementation to get rapid cross-platform availability. Emulators from other parties are most welcome. Be warned, however, that the specifications are not yet stable. Feel free to comment on them!