The 123 combos remind me a lot of the SWToR's "auto" attack. Meant to have a button to push at all times while minimizing the impact of that button compared to other choices.

PVP style combos where it's consolidated to its own button (Which is not the first implementation: The first game I played that used it was Aion, but even then that game hard horrid button bloat) allows for better slot of economy, which in turn allows for Standalone abilities to come back or be added.