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.
Again, there are other PCs with the same amount of storage, and people find uses for them. Put Office on there and a very large number of people would find they have everything they need/want. I think most people will want/need more, but that does NOT mean there's not a market for the 64GB model.
Wait, what problems? I put a 32Gb in my RT and it worked flawlessly.
It doesn't "integrate" with Libraries (and thus doesn't work with Xbox Music, for instance) without resorting to workarounds that you shouldn't have to use. Doesn't mean it's not useful for lots of things without workarounds, or that the workarounds don't exist for other cases, only that it really should be made to work better.