Previously conjurers were awesome tanks and I recommended them to all new players for first mage classes specifically because of their ability to conjure archons.

An archon is basically like being grouped with a healer who knows exactly what to cure for you at any given time. They give you sanctuary when you're fighting, they heal you as you take damage, perfect for exploration. Add to it elementals and familiars and other various tricks and you can solo a ton of areas. The only drawback is if anything attacks your archon it no longer heals you. For PK purposes this is how you kill a conjurer, dirt his archon, hope to kill him before archon kills you.

With the change to mob artificial intelligence, the mobs now know that if they attack the archon while fighting you it can't heal you, and you no longer have the ability to be sanc/healed/cured of maledictions while exploring because all the mobs do things to your archon to bring it into combat. Dirt/trip/bash/etc.

It basically means you have to solo hero as well if you don't want groupmates dying. The only way to not die while tanking is to flee, the mob will break off the archon and it will go back to healing you. This isn't productive for group ranking OR exploration of high level areas. You're going to flee into bad mobs the whole time.