I'll just throw in that there ARE reasons to intentionally mess up combos. Maybe you need to adjust your timing for applying a debuff, so you recast the first ability of the combo to waste a GCD. Maybe you want to intentionally land the final hit of a combo at a later time to burst something reliably.

Arguably, one should be able to map out a fight to at least cast a more effective non-comboed skill to delay by 1 GCD, but if you're having to adapt to something on the fly, having the control available to make mistakes in order to adjust your combo timing is very important.

There are circumstances where one needs to do something fast or in a certain order - averaged DPS be damned. Like if you're solo-breaking gaols on Synced TitanEX as a DRK in a 3-player party. You don't want to start on your first tier combo attack. If for some reason your combo got out of sync earlier, you need to adjust it. Maybe that means repeating an attack once. It may be an overall DPS loss, but it's better than failing the gaol break and wiping the run.

That's just an example from my own gameplay, but the main point is that if you ever get off-sync with your rotation, it may be better to repeat an attack than to continue indefinitely with a terribly out-of-sync rotation.