Loot remains one of the prime motivations for running dungeons. We run dungeons to gear up and run raids, which then gear us up for the next tier of raiding. Better loot lets us perform our roles more easily, makes daily questing and grinding easier, and serves as a status symbol of sorts, as well as clogging up my bank with more transmog fodder. (This is not a post about needing another void storage tab, but man, I really do.) And as a result, for as long as there has been loot in World of Warcraft, people have complained about how it dropped, about how it didn't drop, about never getting the drop they wanted and getting the same drop over and over again. I understand this frustration. The current loot system used by LFR and world bosses like Sha of Anger and Galleon often maddens people with its quirks.
But I tell you now, there will never be a perfectly rational loot system that gives you what you want or need and doesn't give you what you already have.
Standing atop a pile of bones cursing the dagger you already had
Face it -- if you get what you want, you're far less motivated to go back. I don't run a lot of heroics on my main anymore, I might run one a day if I feel like tanking, and even my alt mostly runs the dungeons that have drops he can use. In terms of keeping content fresh, one of the tricks of the system is to leave loot up to fate, chance, randomness to deliver or withhold. This open secret has been at the heart of random loot since the system first rolled out with the game, and has been present for every dungeon and raid ever since. Why doesn't Blizzard make drops more rational? Because it would be a short term benefit for us, but a long term negative for the game as a whole.
There is only so fast that new content can be designed, tested and delivered. Mists of Pandaria is potentially the fastest and best Blizzard has ever been at delivering content, with patch 5.2 already on the horizon a scant four months into the expansion. But even so, we are left with months at a time with the same dungeons, and those dungeons have to remain attractive to players. While it's true that the introduction of point systems (originally badges in BC) helps to motivate returns to these dungeons past gearing, it's also true that people are generally more interested when they know there's a drop in there.
It's better for us if you don't always get what you want
This holds true for LFR and world bosses as well. Actual raiding has a different selective pressure to sustain it - even after you have the drops you want, you're expected to keep coming back for the people who haven't gotten them yet and to keep momentum going for the upcoming next tier - but LFR and world bosses are much more directly sustained by groups wanting the gear from them. A rational loot system sounds good, even to me. I'd prefer getting the sword off of Elegon for my alt than not getting it. But over the long run, it would lead to far fewer people running the system at a rate it simply isn't prepared to sustain. The game benefits from random loot, because it benefits from your being unsatisfied and coming back for more.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.