I'm in the camp of keeping the combo separate but not for a dumb reason like "skill expression", there's not much skill expressed by pressing 1, then 2, then 3.

No, why I want them kept separate is that there exists times when you want to intentionally reset your combo, like if you want to do a 1-2 and skip the 3 on GNB to keep refreshing the shield and keep yourself alive while the healer is hardcasting a raise. Or situations like how you want to reset your combo and take a minor damage loss for better alignment later on in the fight, that's an element ot skill expression.

All that said, I wouldn't be opposed to the option for people to condense the combo into one button for those who want it. As long as it's optional and not forced on everyone.