One of the reasons I'd like this is because, as a healer, I get nailed by bosses casting things that I don't see more often than not because I'm constantly targeting party members to heal them so I don't see the cast bars.

And before someone says "Just make the boss your focus target!", what about adds that cast things that I need to be aware of? It'd be a giant PITA to set each new add as a focus target.