In hindsight it’s easy to see the design evolution.
In Cataclysm, Blizzard listened to the complaints from players that Wrath Heroic dungeons and raids were too easy. They gave us some really great, challenging-without-being-ovewhelming, Heroic-level dungeons. They gave us a raid tier we could really chew, each boss by itself a very satisfying challenge to overcome. It felt like a return to form, and to the days when raiding was respected as the highest challenge the game could offer.
Many, many players complained, having not the competence/resources/time to complete endgame content to a degree they were satisfied with. Blizzard responded to these complaints with the assurance that this was all a temporary problem that would only persist for the first tier of the expansion; that once the next tier was available, the previous tier would be made puggable.
It was a couple of months into the next tier when they noticed that puggers still preferred to live on the scraps of the current tier [Firelands trash runs] that complete their now-tailor-nerfed previous tier, no matter how retardedly easy it was. And so we had a very jarring mid-tier “design shift” to make the Firelands normal modes puggable and only the Heroic modes aimed at progression raiders. And this design continued into Dragon Soul; Heroic modes for the progression raiders, Normal modes for the PuGs and their helpful family-oriented guilds, and the LFR for the retards banging their heads on the keyboard and whingeing that they deserve everything a skilled played can get.
I can’t say I’m completely happy with the current system, but that has more to do with LFR creating once again a system with multiple raid lockouts, so for the first few weeks of each tier, raiders are “forced” to do LFR in addition to their regular raid, every week, to maximise gearing opportunities– the first time this has been necessary since Icecrown 10/25.
I’m sure Blizzard thought long and hard about whether the LFR and the “real” raid lockouts should be shared. If getting loot from the LFR version of a boss warrants locking a person out from receiving it a second time, surely the having rolled on the far superior normal mode loot is an even better excuse to lock a person out from taking loot..?
The conclusion which makes the most sense to me is that they see “forcing” raiders to carry incompetent players as a happy side-effect of keeping the lockouts separate. Because in Blizzard’s view, causing competent players to make the game more fun for [carry] incompetent players is the best result possible.