The biggest issue for me is having to change target completely when you're fighting a monster. Like, for example, if I am playing Conjurer (or WHM), and I am soloing monsters using my offensive spells, with subtarget off I can't press cure to heal myself, I have to completely switch off my target to cure.

My proposed change would be: If you are using friendly spells (cure, buffs, cover, etc) and your primary target is either NOTHING or ENEMY and subtarget is off, you will get a subtarget cursor so you can keep your current target selected. I feel like this change was intended primarily for healers, but also for nukers, and because of this problem I cannot use it.