Defense is useless in hardmode. Bonuses > defense, but raise it as much as you can anyway for when you slip up and actually get hit.
Hahaha, total and absolute lie*. Even full (melee) adamantine + full-up warding accessories makes you nearly as tough as full molten does pre-wall. I haven't nabbed up the hallow set, yet, but that'd make it even better worse. Even limited dodging with minor potion use makes you basically unstoppable
For better raw numbers, melee hallow + 5 warding accs gives you 70 defense, which knocks 35 damage off of everything that attacks you. A ironskin potion knocks another 4 off, for 39. That just shy of halves the damage getting smacked in the face by a wyvern's head does, and reduces the damage from standing on the non-head parts right down to one. Clinger ranged damage? To one. Melee, halved. Laser snail lasers? To one. Corruptors? Damage ~halved. Unicorns, same. Chaos elemental! One damage! Destroyer! Everything but the head! Damage reduced to one! The head cut to less than half! *maddened, borderline invincible, cackling*
Don't want ironskin? Add four to some of those numbers. Still in melee addy? Add two. Anyway, you get the idea. Full defense spec is still pretty frakking incredible. You lose out on some damage (Fairly notable amount vs. a full damage spec, honestly), but... it's viable. To say the least.
*Well, okay, not a lie, but definitely not giving defense its fair shake. Let's run some numbers, shall we!? AHAHAHAHAHAHA
This, oh so very much. Defense is a total godsend, unlike what you think, as getting hallowed + full warding will make chaos elementals just as strong as zombies with medium mode equips.
And being able to tank skeletron prime is nice, as getting full defense makes his prime saw just a minor nuisance. Add to that a bunch of heart statues carefully rigged to wires, and you're invulnerable. And finally, using excalibur along with that defense boost will make enemies bow down to you.
To make it even better, use a warding warrior's emblem to any slot that isn't occupied with anything useful, and you're going to mow enemies down very efficiently.