On second thought, once you start enchanting and once custom drops are in, I can see a reason to use full leather rather than one diamond etc.
Not as true here.
1) You can't enchant books. (to my knowledge as a previous enchanter)
2) You can't combine 2 Efficiency IV books to make an Efficiency V book for the cost of exp or any other known mechanic.
2b) mainly because the anvil is pretty "Broken" and can't be managed or something.
Correct me if I'm wrong @
Kainzo ? Do you have any plans to add these vanilla mechanics into the classes?
The ability to enchant books and combine them for more powerful enchanting was one of the key features of actually removing Efficiency 5 from the old Level 50 enchants, when they made it level 30, and the best you can get off the enchanting table + item is Efficiency IV, Fortune III, Unbreaking III.
However, if you enchant books, and combine them at an anvil you can create "God" picks like E5 F3 U4 or something. I'm actually not sure of the limitations of vanilla, I haven't had a lot of time to experiment without OP on singleplayer.
I'm not sure if you can even do that in HC...
Enchanting kind of needs more love. If possible.