The problem would be that you would guarantee a drop, but what are the chances you are going to get that item you want when the boss might have 3 or 4 items that are assigned to your class/spec. You might have better luck taking the 10x extra rolls at a lower success rate than putting all your eggs in one basket and risking that you get a piece of gear you weren't aiming for. I suppose that such a system might open up a whole new theorycrafting field based upon statistical analysis of where it is most effective to use your coins. Now all the hardcore raiders will complain that they need to get PhDs in quantitative analysis to min/max.
Well either way, the point is it would be considered mandatory.
Let me try this. WoW has a lot of subscribers, all of various interests and commitments and likes and dislikes. There is not system that will make everyone happy. There will always be loot QQ. If you implemented a "guaranteed" loot system or some sort of badge system for all gear, you'd get complaints that it's too scripted, too efficient, too fast, and people would get their gear on a schedule and unsub til next patch. When you introduce any type of RNG into the system, you get complaints from people on the bad side of RNG, complaining that it's too inefficient and too susceptible to chance. So what should Blizzard do?
Ultimately, I think the real problem with the new loot system is its implications more than anything else. Blizzard made a system that tells the player "Look! We can do an internal roll just for you, and it'll pick an item right off a loot table designed just for your spec!" So the player gets their first few drops, almost all guaranteed upgrades, and says "Hey, this is awesome! No more seeing 3 Vanq tokens drop while on my lock and not even getting a chance at gear!"
But time goes by and the number of items needed gets reduced to where you're looking for just one or two items off a specific boss. You use a coin and an item drops, but wait a minute. You already have that item. You wanted a different one. So you think, "Hmm, if the system is smart enough to have an individualized roll and loot table, it should be smart enough to eliminate loot that I already have. And smart enough to compensate for bad RNG."
Wait, compensate for RNG? If you work in more efficiency and more "fail-safes" into the system, you work toward removing RNG from it completely. And a part of MMO play is slowly progressing by increasing your character's power, at least in a monthly-pay one. If it was a one-time charge, sure we could justify a fast gearing system. But for Blizzard and WoW to continue being successful, we need a relatively slow gearing system. Too efficient, and the people who LFR is aimed at will "finish" the game. And why would they continue to play?
I guess I just don't think that the people who are really upset with the current loot system and gearing rate are people who will enjoy playing subscription MMOs.
There are only 2 things that I think should be changed as far as the RNG factor goes-
Tier tokens should be generic- as in a shoulder token is good for tier shoulders for anyone. It sucks to disenchant a tier piece because everyone on X token has it for both specs when people on Y token don't even have it for their main spec.
Bosses that drop the obvious BiS weapons should have smaller loot tables. Not saying it should be a gimme, but a 1 in 17 chance for a hunter to get their weapon (quoted from earlier in this thread) is a bit too steep.