The conventional wisdom in computing devices has always been "Bigger is better."
I'm not sure why things appear to have changed; witness the number of threads where people have asked if it's possible to hook up an external HDD. Apple products historically have a longer life than other computing devices, and the likelihood is that users will collect a huge number of apps, store large amounts of music etc over the lifetime of the product. I could see it being very easy to run out of storage, even on the larger models.
I'm stuck with 16Gb on the iPad, but come phone renewal time, I'll be getting the biggest 4S I can lay my hands on, while also keeping my iPhone 4 as an mp3 player. 64+16+16 should be adequate for a while.
Sent from my iPad using iPF