It's probably worth distinguishing things that are still commercially available -- e.g., Pathfinder, D&D 4E -- and things that are not and have been out of print for a long time. I find it hard to feel too bad about downloading copies of AD&D sourcebooks. I will hardly ever look at them, but I like them for nostalgia and occasionally for idea food. Given that the game has been out of print for decades, and that space is at a premium (I own the old Necromancer's Handbook, but don't have room for it in my current apartment), online downloads are an ideal solution.
Speaking of which, it also annoys me that more companies do not make their gaming materials available on pdf to purchase. This is the age of the tablet and laptop. More and more gamers will be phasing out the rucksack of books that used to characterize gamers. This is doubly true for people like me, who live in NYC, and have to carry everything. If I could just toss it in the trunk of my car I'd probably care a bit less. But, with my ipad I can carry my entire rpg library in pdf, and I can also modify the documents to suit my tastes. I can cut out just the pages for ship rules from Stormwrack, for instance, and just keep them handy or open, even though the book is on my shelf.