For the most part, diamond gear in extreme is coming from villagers. E-books are going down the same path as well. When you can trade carrots and potatoes for diamond gear and enchanted books, the spawn rate of diamond ore is really irrelevant. Not to mention, trading with villagers gives xp, so there's not really a need to farm levels in order to use those books on an anvil.
I'm usually of the opinion that the types of people who will x-ray are the types of people who're willing to ruin the game for others, regardless of their "motivations".
I think that dungeons should be the main source of loot such as diamonds, enchanted books, etc. These things can be acquired through other means, but I'd like to see at least a degree of their attainability to be dungeon-centered.
I don't necessarily disagree with your suggestion, but as it stands, I think it's rather pointless. Villagers will single-handedly do more damage to the value of gear than any current game mechanic. I don't think that we're far off from seeing god-book librarians, and I'd imagine that most people with villagers already have blacksmiths with diamond gear. I hope that you've planted some potatoes.