Just a personal anecdote, but when I play Red Mage and Gunbreaker I have my melee/Gnashing Fang combo tied to shift 5-7 because those are the only only spots I have room to fit them without putting them at a super awkward keybind like ctrl 8-0. Having those combos tied to 1 button would make things a lot less annoying for me because I wouldn't miss the key combos on accident occasionally while dodging mechanics and it would free up more space for extra skills.
Having those combos be 3 buttons doesn't make the job any harder, it just makes it more annoying to play comfortably for people like me who prefer to keep my main skills easily hittable but can't because certain jobs have too many main skills to bind.