This is something I have been thinking about ever since LFR came out.
On multiple occasions the game DOES effectively check your spec.
I'll give some examples:
Ultraxion shards.
The game determins with 100% accuracy if you are a healer. Shards can not be picked up by DPS speced Shamans, Druids, Priests or Paladins.
The achievement "Moon Guard" in the End of Time instance.
Instead of checking for the healer by role the game checks the specs of all group members to determine who is a healer. If someone in a healing spec gets hit you fail the achievement (even if they are not the healer rolewise).
(Sidenote: The obvious trick to do this achievement is to use the dualspec of your healer and then do the gauntlet without a healer.)
The confusing part for me is if they specifically have 2 mechanics to determine what spec you are in 4.3 then why did they not implement them in the loot system?
Did they simply not have enough time? Or did they not think it was necessary?
I mean it's obvious why they didn't convert the LFD loot system to LFR standards since that would have required a retagging of all dungeon drops which in the end was probably not necessary.