Totally agreed. Auto-combos don't work for most jobs, because most jobs have off-shoot combos. In PVP they get around this by giving you multiple auto-combos. For example, Paladin has one auto-combo that does Fast Blade > Riot Blade > Royal Authority, and another that does Fast Blade > Savage Blade > Rage of Halone. These auto-combos can work off each other as well, so if you started out with Fast Blade on the first combo, you could still switch to the second combo to finish up with Savage Blade > Rage of Halone. Add a third auto-combo for Fast Blade > Riot Blade > Goring Blade and you've covered all of PLD's current combos in three buttons, and saved three buttons in the process.
Personally, I don't know if I would like this system. I think I would get used to it, but hitting a different button for each part of my combo is fun, and makes a fight feel more dynamic. I still think auto-combos should be considered as a means to reduce button bloat (especially for the rare job that doesn't have any combo off-shoots), but there are definitely other methods they could implement that are less divisive.