30 GB is still a lot if all you want to do is install a few apps, browse the Internet and watch movies and videos streamed from YouTube and Netflix.
That's true, but realistically the reason for getting an Intel based tablet isn't for only those types of tasks. If you just want a consumption device you'd be better off buying the RT or a much cheaper Android tablet which should both have very good battery life. People who buy the Surface Pro probably want it to do more than just that so the 128GB model is almost a necessity for them.
If they fix the problems people are having with getting SD cards to integrate into the OS more like an internal HDD or SSD (which they've promised) then the 64GB model might be OK because you could store all your large videos and photos on a 64GB SD card.