So, copper isn't completely useless:

• Smelting it gives a nominal bit of XP. If you have an auto-smelter setup with a switched output hopper so that you can collect banked XP, it can save your ass when trying to enchant or repair gear in early game, especially if you're smelting with lava buckets (since they're renewable anyway). Once you throw Fortune III into the mix, copper is absurd for this.
• Motherfucking lightning rods. Do NOT underestimate them. Building a villager trading hall? Make goddamned sure it has lightning rods. Villager breeder? Lightning rods. Only you can prevent villagers getting turned into Margaret Thatcher.
• Copper blocks (and slabs) make a nice alternative to stone for things where you just want to use a lot of throwaway blocks, e.g., for spawning pads in mob farms. Endermen can't pick them up, they're opaque, and you have access to plenty of them. They also have a nice grid texture to them (similar to iron, smooth stone, or polished diorite/andesite), so that can be helpful on complex builds where you want to be able to eyeball the dimensions.

Other than that, though, yeah. Pretty damned useless, and only exists as a gateway to inventory management hell, not unlike the overworld stone variants.