The PvP system uses simplified combos because they want the match-determining factor of PvP to be strategy rather than rotation.
The roleplay battles use simplified combos because you can't expect the average player to know how to play all twenty jobs just so they can play a one-off story battle well.
Outside of PvP and outside of roleplay battles, you have the leisure of working out a control scheme and hotbar layout that let you execute your job's rotation well and there need to be some error points somewhere. "But it's not hard to hit 1 2 3." Yeah well the point is you can do it wrong, and some people will.
