Secondly, this provides an opportunity to enable versatility at lower levels. Rather than having to trudge through low levels with only a few core actions, the horizontal progression this provides would make combat more interesting without simply being a boost to your main role performance.
Thirdly, customization of your character's play style would improve the game in many ways. Characters would feel more personal beyond their appearance and invested time, especially if the actions stand out visually or affect the flow of combat. There would be more to team dynamics than classes and levels. While you could still expect the healer to keep your hp up and the tank to aggro, there would be more to optimizing the battle than having everyone perform their duties well; for example, two secondary supporters might collaborate to buff and debuff just in time for the dps's limit break.