What you seem to not be aware of is; the CD of Force Barrier. It is THREE minutes long. For three minutes, the sorc/sage has no CD's that mitigate any damage whatsoever. When being focused by 2+ people a bubble/insta-heal isn't going to help; versus other classes that have DMG mitigating CD's the last for an extended period of time.
After a sorc/sage gets out of FB, sure a tank can switch guard to the sorc/sage in a preemptive defense against focus fire on the sorc/sage. But do you expect the tank to keep guard on this DPS sorc/sage for the entire 3 minutes that FB is down? Do you expect the tank to stay in range of the sorc/sage to keep guard in him whether it be by his conscious movements or by the enemy team utilizing a pull/knockback for a whole three minutes?
- Pulling the tank out of guard range.
- Forcing LoS issues upon the tank/heals.
- Mezzing/stunning the tank.
- Mezzing/stunning the heals.
Any one or combination of the above ^ dashed statements over the course of the Force Barrier being on CD will lead to the sorc/sage being destroyed. The enemy team has THREE minutes to carry out any strategy/tactic that can accomplish this; very much so in the favor of the enemy team (Not in favor of the team that the sorc/sage is on).
This scenario can also be applied to Mercs/Commandos and Powertechs/Vanguards but they can withstand focus fire better because they DO have DMG mitigating CD's and they also don't get locked down by roots/snares like sorc/sages do since they have a CD that is almost always off CD that makes them immune to roots/slows/pulls.
This scenario can also be applied to ALL other classes but it also will not work NEAR as well as it does against sorc/sage because all other classes have DMG mitigating CD's and they also have better escape CD's whether it may be the roll for the operative and sniper, stealth for operative - assassin - marauder, superior DMG mitigating CD's found in marauders and Juggs that easily counter focus fire for an extended period of time, etc etc etc.
Any competent team will capitalize on a DPS sorc/sage once they have exhausted their Force Barrier; there is absolutely no counter argument for this, and any tactic/strategy by the team to keep the DPS sorc/sage alive will ultimately fail against a competent team.