It should be noted (for anyone coming here to find out), that the "Illia" bug has been fixed - with a catch: If you haven't acquired her as a follower yet (she's the "good" witch you meet at Darklight Tower), she can be dismissed as a follower, and reacquired whenever you want (I verified this by loading an old game-save before I met her, and playing out her quest again). However... if she is currently your follower (before this latest patch, 1.4) or she has been dismissed... you still can't have her follow you again! 
I am pretty far into the game now, so I don't know if I'm ready to fall back that far into one of my older game-saves, so I can have her as a "reuseable" follower. At least if you start a new game, now (or a game-save before meeting her)... you won't have to worry about being able to have her as a follower whenever you want!

I am pretty far into the game now, so I don't know if I'm ready to fall back that far into one of my older game-saves, so I can have her as a "reuseable" follower. At least if you start a new game, now (or a game-save before meeting her)... you won't have to worry about being able to have her as a follower whenever you want!
Just tested this and it is completely untrue. She is still broken, even if you've never spoken to her before. I just did the same thing, reloaded an old save before ever discovering the tower. Cleared to the top, finished the quest, had her join me. Dismissed her, and she immediately begins with the "Need something?" response and no dialogue options. If you managed to get her to work for you, great, but it is likely unrelated to anything done in this patch.