The original reason for the sync system being this way was to keep higher level players with access to more abilities from being way stronger than lower level ones, even with their stats synced.

Take a look at SWTOR. You can level sync down to lower content in that game. you get to keep all of your abilities and just your stats are lowered. Problem is, your entire rotation/skill tree is based upon what abilities you have unlocked. So the result is that a higher level player will still do 2-3x more damage than a lower level one, even if their stats are synced down.

The system that FFXIV puts in place removes this disparity. Compare a lancer who does and one lancer who does not have access to full thrust. The damage disparity is massive since that skill completes a 3 hit combo and has huge potency.