I am much more proactive about limiting my acquisition of digital media (books, music, etc.) than I am of physical media. Alas, owning the Library of Alexandria means that I have to be the Librarian of Alexandria. Searching ships for new texts, transcribing those texts for the library, sorting through them, and dealing with the rest of the mess. Well, maybe it isn't quite that bad but I'm sure you get the point: the more you have the more you have to deal with. Even though wonderful tools like Calibre helps take off a lot of the load, it doesn't solve the problem completely. Since it is far easier to let digital collecting go unchecked (less visual feedback, fewer space constraints) it is easy for the librarian bit to get out of hand.

The other thing is that I don't understand having a 10 year reading collection. Tastes change, and it is nice to get newer titles when they come out. And let's face it, there are free books coming out all of the time (old public domain stuff is digitized, new stuff is created for the public domain or under creative commons or gratis). On top of that, formats are being improved all of the time. An old collection is stuck in the past. Do we really want to read ASCII Gutenberg titles because that was the only format they came in when we downloaded them 10 years ago?