Well don't look at me, I didn't figure it out until then and I already had two level 50s.
I still get a chuckle when people don't have target of target turned on (especially tanks), but then remember I did not either on my first toon until we were about to pull Bonethrasher the first time I was in KP.
I can't imagine not having target of target on. Helps me on my dps to know when I've pulled aggro, it's vitally important to my tank for obvious reasons... Not so much on my healer tho. I've often wondered how interesting it would be, to have an option to see "Target of Target's Target" or maybe even "Focus Target's Target"... i.e. I am on my healer, and I have the tank targeted. I see that the target of the tank, is the boss. What if i could see when a dps pulled aggro, by seeing that the boss (the tank's target) switched to another player. It would let me anticipate sooner that the dps was about to take a hit.
Maybe it'd more confusing than it is worth, but who knows.