would diamonds (or lion's eye diamonds) have any special meaning or a high value if they were found in abundance all over the place? How many practical purposes (or deck archtypes) do they truly usefully serve?
Diamonds ARE found in abundance. Their "scarcity" is artificial, created by a monopoly by the DeBeers corporation. They aren't special at all, and are intrinsically pretty much worthless.
Diamond (usually lab-made) is used as a hardening agent for many types of tools (often drill bits). A drill bit with diamond in it can cut through much harder materials than your standard steel drill bit.
I know, that's not the point you were making, but if you're going to illustrate a point, use an example that doesn't directly contradict your point.
Them putting these in the list is honestly, what I think is, them looking to push set boosters more and likely trying to get rid of draft boosters.
They won't get rid of draft boosters. Limited is a big enough part of Magic that the monetary losses would be significant. If even a product like MH2 is designed to be drafted, then drafting is just too much of a moneymaker to get rid of.