Its not worth purging all of those stacks in 10m. Just wait for it to fall off.
It takes 1 GCD assuming a good dispel group makeup. We have 2 mages, a shaman, and 2 priests. This means the shield is gone instantly.
The only time I wouldn't worry about dispelling is if you only have 1-2 offensive dispels in the group. If you ignore the shield, your melee have nothing to do and your ranged might be able to blow up the fixates for a few seconds if they weren't before. You could plan for this in your strategy: just kite fixates and then kill them all when a shield pops up.