Depreva touched on this previously and I would like to put more emphasis on it.
Once a sorc/sage uses their Force Barrier; nothing is stopping the other team from focusing the sorc/sage until they are dead. Sorc's do not have any defensive CD's that mitigate damage, unlike all other classes.
When it comes to multiple people focusing a Sorc - an insta-self heal is not going to help the Sorc, regardless of how big the heal is. At least the Merc has a 25% DMG reduction defensive CD (correct me if I am wrong I do not play the class.) So as the attackers that are focusing this Sorc, would you rather have to DMG through the Sorc's 6k heal or would you rather have to DMG a target that is forcing you to do 25% less DMG for an extended period of time? Clearly as the group of attackers; you would prefer that the Sorc has an insta-self heal and not a DMG mitigating CD.
Step 1 - make the Sorc/Sage use their Force Barrier
Step 2 - call out to your team that the enemy Sorc/Sage has exhausted their FB, so they have a mental timer as to how long till the Sorc/Sage's FB is back off CD.
Step 3 - all team tactics/strategies immediately turn to having the Sorc/Sage killed.
Step 4 - GG.