The Retribution 4pc does grant a self-buff that is consumed by the next TV, and does not stack. Wasting some procs is expected, and balanced around. Yes it is a single-target benefit that won't increase DPS on AE fights. We don't want to be in such a limited design space (we're talking about the 15th tier here after all) that every set bonus must be super generic and work in every situation. If anything, we'd rather the Ret bonus be even higher for single-target and paladins will just be stronger at single-target and weaker at AE once they have the set bonus than they are in other tiers. Set bonuses are one of the few chances we have to change what your character does (within reason) aside from all of your numbers just getting bigger.