/bt from my experience will only allow you to target mobs that are "red" while in combat.

If you are fighting something and another pops or adds, and you are under /battletarget on you will not be able to target that 2nd mob unless you use an AOE or use a subtarget skill like provoke or a spell.

It's a pain in the ass when in a party and the above situation happens.

A simple solution would be to allow us the option to have a targetign priority style that target closest to furthest mobs from left to right or right to left.