I travel a lot. (and I live in Hawaii with a lot of volcanos, waterfalls, and other nice things to look at.)
The iPhone 4s has a really great camera and can do 1080p video.
If I'm in Kyoto, or Shanghai, shooting video of cool stuff, I want to make sure I have a lot of room for a few hours of 1080p HD video. I know iCloud will help, but if I'm roaming internationally, I don't think I want to be messing with 4GB movie files. I'll keep them in the phone until I'm on WiFi or in USB striking range of my Mac.
That's why I got 64gb.
Honestly, before I saw the details of the new camera, I was planning on getting just 16gb and using iCloud for most everything.