As an added note, you get LoS as a BLM unless you are facing the target as well. Not sure if this is the case for all classes that have attack spells, but you also get LoS as WHM with attack spells if you aren't facing the target, though with healing spells you can heal the target no matter where they are unless a "solid" object is between you or they are too far away. It's just the mechanic of the actual spell or mob.

I know before 2.2, you could hard target a casting mob and literally run circles around around them at the right distance and they would never cast a spell that would hit you. They changed that and other spells so that they would hit regardless, but kept certain ones LoS (like the III spells on the boss in AK and as mentioned above, Garuda's stun attack).