It's not just about simplifying. It's about making the spec play in a way that's not too complex or too simple. Retribution Paladins don't have a whole lot in terms of rotational choices beyond choosing when to refresh Inquisition. So there's a good chance Ret was seen to be "too simple" and they chose to add a bit of complexity in how the transition from single-target to AoE is carried out. Many other specs have to change their rotations to do AoE, Retribution Paladins would have to spend a GCD on it.
I admit I don't know much about Shaman. Can Enhancement plan ahead and use that GCD to change "gears" while moving towards the adds, for example?