You could ask the same about the Blu-Ray drive. What does it add? You can watch movies with the current DVD player. You could just watch better quality movies with blue-ray. So it's not like now you can't, but it would improve.
More ram and speedier processor adds speed which you need for newer applications and to get the mini more 'future-proof'. An upgraded graphics card opens up a bunch of new possibilities like gaming, maybe 2 DVI outputs (or DVI + HDMI)...
I'd also like Blue-Ray of course, but first the mini needs CPU, GPU, wireless N updates. Blu-Ray would just be a nice plus for me, but probably would make it more expensive than I want.