My personal opinion: I think the idea of targeting lines all over the place would look terrible.
Some players are already unhappy with the aggro icon (and mob level indication that SE are planning to introduce) on screen as it takes a lot of the feel away from the game.
FFXII was actually the only one I never finished- I didn't like the game and especially did not like the targeting lines.

That being said as a compromise (and if people really want them introduced), then a red target line from the mob/s to the player with hate would be ok if it could be toggled on or off. I don't see that there is need for a "blue" ally target line showing which mob is being attacked; that really would be brain melt with a big alliance.