Maybe it's a bit redundent:
Because we already have the Ctrl + T function
Thing is though I bet you've been in a situation where you wouldn't care much for attacking the prioitized target (ex. a Anti-Caster build isn't going to care much for killing a Warrior)
Or there's multiple prioitized targets and, sometimes, you hit the T key after someone's called out another target and the first one isnt even down, and in thoes 'oh so lovely' situations where things get frantic, you might not notice.
For a practical example:
Let's say I'm running Hundred Blade + whirlwind attk + sun/moon slash on my warrior and i ping my build. If the necro in our party see that and takes mark of pain, it makes it easier for him to put that skill right where it'll do the most damage.
And im sure you have ideas that would work beautifully if you could see who your party member is targeting whenever you wanted to, and they could see who your targeting. Which would become more useful the more players you have in your party
It be a throwback to Thackery + Miku's combination (the one where you insta-kill people

-------------------------------
I say only have it able to see what my ally is attacking only cause if you can see who your enemies are going after, it would be a double edge-sword in PvP. You could target another enemy to mislead them but could also see who your eyeing up and warn them over voice chat, which is why I say: Keep it to Allies only.