It's better than pressing one attack over and over. You're also describing a dummy situation where there is nothing to do but repeat your rotation. The variety of buttons creates additional depth and complexity in real encounters where you might need to switch targets, position for a mechanic, or burst through a particular phase. Those are things that you would want to plan and possibly adjust for.
Healing on the other hand has no depth to attacks and no depth to healing when healing becomes trivial or completely unnecessary.
Fire 4 could be looked at as filler, but that's not a bad thing. Filler is a problem when it's not so much filling, but is the entirety of your rotation. Xenoglossy, Despair, and Flare Star are all more important than F4 and F4 itself is constrained by a timer. A BLM is trying to maximize F4 casts while maintaining the AF timer and also managing other resources like Triplecast and Firestarter that can be used in different ways. Triplecast can be used immediately on CD for DPS or held for mechanics and likewise Firestarter can be held for transpose for damage or used to extend AF.
Compare to WHM which has Glare as its filler. If there is no healing to be done, Glare is cast repeatedly and only broken up by the occasional DoT. There is no resource management with Glare nor is there any constraint on its use outside of MP which can safely be ignored most of the time. There is the option to spend lilies for Misery which can grant a small gain in damage but there isn't much in the way of decision making unless healing is required somewhere. Unfortunately healing requirements can be so low, that Assize pressed on cooldown is sometimes enough to heal through fights by itself. BLM has an engaging rotation built around a filler skill. WHM has a filler with nothing else to go around it which makes it feel much worse.