Emphasising this incase it was missed.
I'm not a fan of staring at health bars instead of the field in any non trivial content. Setting the health bars to only show when party members are less than 100% hp gives you a much more obvious cue that they've taken damage, and watching players themselves rather than some bars makes it easier to preempt incoming damage and/or mistakes.
It's actually really useful in quite a few higher end raids, allowing you to keep tabs of more casts than you might be able to do easily with just Focus targeting. Shinryu EX's wings are a great example of that.