Demon armor in tbc was useless, fel armor gave both SP and healing received.
In wotlk they changed fel armor to give hp% regen instead of +healing received. So you did swap demon armor for defense.
In cata it was also fel armor for SP, and healing received on demon armor.
Oh, the way that Indead was speaking, I figured they merged the effect into Fel Armor in Cata. Was a while since then so I didn't remember.
So yeah, that's one thing they successfully removed from WoTLK/Cata times that was actually meaningful and I guess "fun" to do. I remember swapping to Demon Armor whenever I was about to get Kidney Shotted and it making all the difference in the world f.ex.