We saw people several turned away several times at AK in December. Maybe because it was super crowded. Lady ahead of us was fuming that there should be a sign (which of course there was). Also saw it once at Epcot, but we never really use it at MK because it usually moves slow.
AK in december (and early Jan) was the first time I have seen people truly turned away from the AP line who did not have passes. I go fairly regularly and had seen them asking but not really enforcing. One day lines were insane (it was the last blockout day in Jan) and there was not one person in the AP line....the blockout days are definitely the real sweet spot of the perk if you have platinum or higher passes.
The idea is nice but where they place the lines and how they (don't) enforce AP makes it only sometimes useful.