Well im actually having decent results playing aff in s3 with beguiling. I still prefer demo, but sometimes i feel like playing aff and i have timed a 14 key with it, so its definetly viable for most players to play imo, can't really use MDI as en example of whats viable unless you push keys thats really high.
Overall it's a fun spec with beguiling and i'll probably keep playing it since it's my raid spec atm, and i will prio getting azerite for aff (i refuse to pay absurd amount of gold to reroll traits all the time)