Yeah a defense boosting Berry with Synthesis could be a decent option to constantly raise a defensive stat, but even if you become unkillable from the special side, you are still easily walled and destroyed by U-turn or Fighting moves so i don't know if it will get any use. But hey, all this is theorymon, and the only way to see it is to wait untill we try it. And as you said, and i have mentioned in my spread's description, Lum and Sitrus Berry will be the most used Berries.