I used to use the "I like having the actual Disk/Book/DVD" as one my arguments against piracy. I'm still anti-piracy, but I am finding the need to actually own these things something that annoys me about myself, if that makes sense. Why exactly do I have to build this DVD library? Especially when most of these movies I wind up only watching once. Sure there is a convenience when I get the urge to watch one of my movies, but for the most part they go untouched. Sometimes it works out having them. I probably have half of Elvis' movies, and my girlfriend and I decided to watch at least one a week in chronological order, which means buying around 15 more movies to complete the collection. But I rarely watch special features anymore, I am less and less caring about how crisp the audio and video is. Hell, I still haven't even bothered to switch to Blu Ray. I really have no desire to. If I can live with the quality of a Rudy Ray Moore DVD, I can live without seeing Nicholas Cage's nose hair in high definition.
As far as books go, I have become for the most part a library guy. It helps that my girlfriend works at one, and can get a new release for me pretty quick. But once I'm done with the book, be it something I loved (11/22/64) or hated (1Q84), I don't feel the need to actually own it. By the time I want to read it again (which I rarely do), it has been out long enough for me easily check out or find for a buck at a seconhand store.
Music is a little more tricky, mainly because there is so much stuff I listened to in the last 20 years that is hard to find, I feel for that reason I should have that physical copy as a back-up. But as far as buying music, I rarely do anymore. I've gotten old, I think, where I just do not like any new music I hear. I just listen to what I have on my ipod, or just try to find new stuff on Spotify. I still keep all my vynil, even though I don't have a working turntable.
As far as how much of it I have, I have way too much. So much that it annoys me. But for some stupid reason I can't find myself to get rid of any of it. And there are obvious exceptions to the rule. Hell, I got a couple of DVDs for Christmas (Abar: The Black Superman and Madman) and love them both. I also have a book coming from Amazon right now. It's the hardcover Skyrim game guide, but that is one that I can't find in a library, and I will get a lot of use from.
But (with the exception of videogames) the older I get, the less and less I care about owning stuff.