yeah the diamond community is pretty bifurcated over whether diamonds should be saved or used outright. personally, I'm in with the "if you have 'em, use 'em" crowd -- not for anything like armour, since that's a huge resource sink, but you're going to need a diamond pick sooner or later, and a diamond sword is incredibly useful. I generally just use iron or stone when I'm just digging around, and diamond for mining ores and obsidian.
shovels are hella speedy no matter what you're using, but for that reason shovels of lower-tier materials get used up incredibly fast. diamond stuff'll let you just dig away without a care, so I guess it comes down to how much it bothers you to need to switch in new shovels or run back home to craft another batch.
(and yeah, using a shovel to attack or a pickaxe to dig soil or whatever takes twice as much of the tool's "health". it's pretty annoying, particularly when you're cutting down trees since sheep will run right in front of you and get hit with your axe, and every hit on a mob deducts health from the tool. as opposed to blocks, which only deduct health once destroyed.)
shit there is no way the diamond subject is so important that I needed to type up three paragraphs. that is just
as fuck of me.