The point of giving the freedom of movement and not being stuck at the same place is so no one DIES at the same TIME in an instance. The thing is, if healers, rdps and whoever are all bunched up together and for example, a skill targets one like Time Bend.
Time Bend from Lisk is a target oriented AoE. Meaning if person A, is standing with person B and person A gets targeted by the skill, person B also gets damaged and debuffed by the same thing. Now, if a mob has the same principle but the debuff is much worse, say Stun 10 sec -- it can be party breaking if its a spammed debuff. Like, 2 Lizard mobs in Circus Insane.
Ergo, if your healer is standing with anyone in close enough, theres a chance the party fails. That is why I gave a range of movement. BY ALL MEANS stand with whoever you want, but i am not promoting that everyone would be bunched together in one place.