Well, I was wondering about this too, mostly because of music to listen to while I read. I listen to 4-5 weekly DJ shows that are available as free downloads from their websites or iTunes. (For instance, Suzy Solar's weekly radio show, http://suzysolar.com/suzysolar/category/tracklists/
But they are about 160-220 megabytes each. So each week basically I have a gig of music to listen to. However, I often don't have the time to listen to everything in a week, so I like to catch up when I can. Being able to load 3-4 weeks worth of shows on it would be nice. Instead of probably 1-2 weeks.
And really, 3500 books is a lot, but probably only 1/3 that I've read in my life, and I'm just barely middle aged. If you read for 2-3 hours a day, you can pretty easily read 400-500 a year. Then again, that would still take several years to read for most people, and if you can't change the books in years, well.