Im pretty sure thats been mentioned before.
It would make it too easy for people to AFK train on weak monsters. because they could sit at a spawn and go afk and all the monsters that dont attack everyone else would attack them and they take very little to no damage.
There is always a solution we can think of. So it can have a percentage of poisoning effect or something similar... One will not dare to use it AFK. (Should be an expensive item so it can be lost when died)
There will be a good option to have a perk, cape or an equivalent equipment, to remove ignored monsters, and all monsters would attack you whatever your combat level or defense level is.