You know the "defensive" line gear? Only 5* gets specialized defense, but the rest of the alchemy line and Ancient Plate have pure normal defense, which is non existent in T2 and T3. Normal damage is there, but the armor doesn't provide enough to negate anything really. Ironmight has stun resist, but no pierce dealing enemies deal stun; the set really has no place anywhere. On the other hand, Volcanic Plate has stun AND fire; 2 natural status resistance, both of which are common by the elemental damage dealing enemies and makes it quite at home in FSC. Ancient Plate set is neato that it has the highest defense and natural bonus health... but still it doesn't protect you from much in T3. Maybe T1, but not T3.
So my question is, what situation would prompt you to use any of the plate gear at all?
Is it really a crown sink waste of space?
Normal damage is not non-existent in Tier 2 and Tier 3. There's tons in Tier 2, and still a fair amount in Tier 3. For example, in Firestorm Citadel, the trojans, slag guards, and Vanaduke's mace all inflict lots of normal damage. That's why people use Volcanic Plate Shield. (That, and the fire and stun.)
That said, you're right that there's very little use for these armors. Spiral Knights encourages a fast, dodging play style, rather than a slow, tanky play style. This has been discussed in many threads, in Suggestions and elsewhere.
One possibility: Do rocket puppies inflict a lot of normal? (The wiki says normal and elemental.) If so, then resisting normal could help in arenas where there are up to 12 rocket puppies at once. But even then we'd need a test to see how much difference all of that normal protection makes.
Also, please remove the extraneous copy of this thread.