So what should it be changed to then, if we want to accomplish what OP is talking about?
Depends on your party composition and play style. It's not a miracle cure for mobs getting pass the tank though.
I have Solas on Defend Cassandra so he targets his barrier at her, I haven't seen him run into melee to throw a spell yet. Nor have I seen Varric run into melee with Follow Cassandra (he attacks her target(s)). I've seen plenty of mobs running up to them to whack them in the face though, but that's a tank aggro problem, not a running into melee.
Also, I always send Cassandra face something at the start of fight to be sure she gets aggro (she will run up to it by herself) and will attack that until it dies then switch to her behavior (which his defend my PC).
Otherwise, there is always the option to disable AI and manage everything by hand.