That you can right click your character frame and one of the options there (among "leave party", dungeon difficulty, pvp toggle etc) is "loot specialization".
Perhaps it was not there before, perhaps i was 100% blind (totally possible), but i have only JUST discovered it.
After having spent forever gathering offspec gear on multiple chars in LFR/flex etc by ninja-respeccing before boss pull, because i didnt need mainspec gear but had no gear to permanently run offspec yet.
Soooo apparently you can use your mainspec while still getting loot for offspec.
Today i learned something new.