Branching is sort of the reason why. For example, WAR has more than one conclusion for it's Heavy Swing -> Maim combo. PLD has more than one conclusion for it's Fast Blade -> Riot Blade combo. MNK has TONS of variations, as does DRG and SAM. There are also reasons not to want to progress in a combo... for example, I might not want to go into Mythril Tempest from Overpower while tanking as a WAR. I might just want to spam two or more Overpowers. It's a concious design decision, because in PVP they just do exactly what you want and turn it into one button for the whole combo because you only get one combo anyway.
Monk... I don't know monk. I accept it's more complicated, but things still seem possible here. Basically, I see a greyed-out button (or "only good if after a combo" ) a waste of hotbar space. I'll bet controller people are even MORE frustrated by this.
Oh and Trusts. Great while getting to 80, but what they do after you finish MSQ is just idiotic. I think it's a terrible idea, but I won't spoil it for those who haven't gotten there yet.