When I was a kid / teenager / young adult game books were meant to be used, meant to be marked up - heck, even torn apart sometimes. My copy of the DMG had perforated pages in the back for easy removal and reference.
How many today would rip hose pages out of their brand new, $40+ rulebook?
Back in the day I would actually write in the modules as I ran them, or even made notes prior to running right in the book itself.
Marking off hit points, scratching off spells as the NPC cast them, making adjustment to the adventure prior to running it... this was common practice.
Now, we have adventures that are presented as "over 250 pages, delivered in 100% full color, tightly bound with our infamous stitched-bound hardcovers -- or secured by genuine leather covers for our serious collector." Who is going to actually write in the above $110 hardback? (I'm not even considering the leather edition)
Is a book of the above quality even meant to be truly read, let alone used? Is that why we have PDF copies of such, so we can put out collectible on the shelf with the hope that it will appreciate in value and impress our friends and escape the touch of human hands?
I'm not knocking the high quality work that many companies produce, but when did we change as a hobby?
I find myself loath to write in the POD copy of paperback copy of Dyson's Delves, a book specifically made for one to write in.
I may just need to stat out a dungeon in that book, in ink, just as a form of therapy ;)