Can one ever really justify taking Barrier?
Barrier rarely ever works to save your life on those who MIGHT consider it useful, but its main issue is that it only has one effect: shields you. No extra speed like Heal, no Grievous Wounds like Ignite, no slow like Exhaust, just ONE effect. Even if its cooldown is shorter than some of its contemporaries, the effect just washes out very easily.
Maybe if it absorbed any damage that went over the barrier's limit it'd be useful, but Riot would say "BROKEN" in a heartbeat because of how strong it would be against burst.