There is no "one" useless item in minecraft. Many items in Minecraft don't serve much purposes. Like:
-Dead Bushes and Ferns. (Unless you like decorating like me :))
-Poisonous Potatos. Like, duh it's not like you're going to eat it.
-Rotten Flesh (Unless you run out of food).
-Gold tools and armor. (Unless you enchant)
Those are all the ones I can think of
I still use Iron Pickaxes. When I go caving I use my Iron pickaxe to mine Iron, Gold, Stone, and I use my Fortune Diamond Pick to mine Redstone, Lapis, Coal, and Diamonds
How many do you go through though? A diamond pickaxe with Unbreaking III is equivalent to 24 iron pickaxes, and even the pickaxe I use (Efficiency V, Fortune III, Unbreaking III), which can only be repaired one diamond at a time for 37 levels, is still 6x more durable per repair (i.e. 1 diamond gives back 390 durability, Unbreaking III increases that to 1560, basically a new unenchanted diamond pickaxe of uses) and I can easily afford to use it as my only pickaxe, getting plenty of XP from mining and killing mobs (I even use it to mine iron/gold, stone, and rails, never mind occasionally hitting bats/mobs (-2 durability) with it).
I'll typically repair it 2-3 times per play session (2 play sessions per caving session), which is 12-18 iron pickaxes (24-36 per caving session, compared to 4-6 diamonds; even the 31 iron for a new anvil every few caving sessions is far less than the iron needed for iron pickaxes); I also find about 10 times more diamonds than I need (including repairing other items) and diamonds are hardly useful for anything else besides tools (a few for an enchanting table and a jukebox but that's it; nobody wastes diamonds on fireworks in Survival).