My biggest concern with merging the basic combo into one button is the lack of choice when it comes to your attacks. Sure, you hit 1, 2, 3.
But often times the middle ability offers either a heal or MP or something.
In the case of the Paladin for example, if my team is struggling or half dead and I need to enter a survival mode, hitting 1, 2, 1, 2 over and over gives me a lot more extra MP that I can then use to heal myself. Once topped up I can go back to the standard rotation and start focusing on damage again until I need to heal and regen MP again.
Consolidating that combo into one button takes that option away entirely. Whether or not people think MP generation survival mode is dumb or cleverer is irrelevant. Simply talking away the option because you don't like pressing buttons isn't the best course of action.