It's an issue of people thinking they're doing better than they are. You can raid shield spam any fight in ICC and logs will freak out and say you're amazing and no one can keep up. You'll get 7-10k HPS on every single fight. The issue is that you'd be exploiting meters, rather than taking advantage of the mechanics to play well.
Marrowgar: 80%+ of the fight has no AOE and a maximum of 6 good shield targets.
Deathwhisper: No reason to raid shield at any time unless you have extremely bad interrupters.
Gunship: No reason to raid shield ever.
Saurfang: No reason to raid shield ever (post-fix
).
Festergut: Raid shielding works well on half the fight (0-1 inhales).
Rotgut: No reason to raid shield unless doing his achievement.
Putricide: Raid shielding only worthwhile during P3.
Princes: Raid shielding only worthwhile when Valanar is empowered.
Blood Queen: Raid shielding only worthwhile after third set of bites and during her air phase.
Dreamwalker: Raid shielding only worthwhile if you have bad melee DPS.
Sindragosa: Raid shielding not worthwhile until P3.
Lich King: LOLRAIDSHIELDNOW.
ICC fights in general do not favor raid shielding with some exceptions. Others allow it for about a third of the time, some up to half. There is no fight that favors raid shielding as much as Lich King in the entire game and basing your gearing purely on him is silly. Having some regen gear to swap in on him? Great idea. Tailoring your gear for raid shielding in general? Horrible idea. Disc does very well on all of the above fights. Unless you just blindly spam raid shielding, which is what you're required to do if you want to ignore haste.