I'm a firm believer that nature abhors an empty hard drive (or flash card). ;-)
I'm the sort that has bought the Black Mask collection 5 times (Rocket ebook, MS Reader, MobiPocket (twice) and most recently, the 25,000 volume version in ePub.
Plus, I downloaded the Gutenberg library DVD image and ran the prettifier on it to produce my own LITs with proper Metadata (a weak spot of the Black Mask Collection.)
On the commercial side, I've bought all the BAEN webscriptions and a few dozen volumes at Fictionwise, Amazon, and Powells, over the years.
Bottom line is that I can arguably claim to legally own between 25,000 and 40,000 ebooks. And they all fit on one 16GB flashcard.
While I would like to think I'll live long enough to read all of them I don't expect to. Its not my intent. The idea of having such a collection is simply access; that if on a moment's whim I'm in the mood for some SF, Fantasy or classical theatre, I can just hit the right directory on my ebook reader and get on with it. No muss, no fuss. Just do it.
To me an ebook reader is more than just a book reader, its a personal library.
(Mind you, I do intend to make a serious dent in it before I'm done, say 10-20%. That much I can do.)
Plus, I'm covered in case an asteroid impact ends civiization as we know it.
A while back Jerry Pournelle said that the day would come when we would not just buy individual ebook volumes but entire libraries (on disc). He even talked of the "Library of the Month Club". That day is pretty much upon us, no?
If not here, real.soon.now.