depends on gear, my bark can add up to 800+ armor on my regular support build.Sheild of bark is a great skill, on boss runs (Aggy, pyrus) if a druid has bark maxed they could add 450-500 armor reducing the hits taken by then tank making it easier for druids to heal.
i do not believe bark should make skill less interruptable as druids are quite strong already, the interrupting is actually one of the things making our class not being OP, as i believe it would be when we have for example a 48 (my) bark and barely get interrupted. as except taking low hits from the mob, it would also not affect my heals and buffs, therefor making druids OP against other classes. druids are already a necessity for any 100+ lvling group and certainly on any bosses.