I can think of reasons-- like gemming/enchanting T6 for DPS rather tanking, running up against the soft/hard armor caps, etc. Sure, if you're the MT you might not make that tradeoff. If you're occasionally an OT and occasionally DPS, it might have a perfect spot even at endgame.
On the other hand, two pieces of arena gear and vet's bracers just about get you to the defense cap if you're looking to maximize DPS or threat while maintaining tanking ability. Hyjal rep ring instead of armor ring,
[Telonicus's Pendant of Mayhem] instead of Juggernaut, DPS or Badge/Moroes trinkets... And at the very least, they provide some nice stam for those magic damage fights.
It really depends on your situation with gear. But the helm, shoulders, and gloves aren't terrible differences from T6, and if you aren't deep in BT you're comparing to T5.