It isn't just for fun. That is certainly not the case.
I recently built a comptuer with 8GB ram for a multi media PC. One of the things it is use for is video encoding and photo editing. Adobe Lightroom 2.0 64bit will use ~3GB ram just for itself, the transition to 8GB from 4 was a night and day difference. Raw Photos 10 megapixils and up in lightroom really scroll and zoom nicely now.
8GB is the way to go, there really is not reason not to. Media Center service when running and recording can be kind of hefty in ram useage, so can the simple web browser. Vista and 7's superfetch will also make good use of it.
Here is a screenshot of that machine.
That is just desktop usage, Vista making use of all that ram by caching programs. Only 48MB free. That ram is not wasted.