In order to make button consolidation not a no-brainer, each step of a combo should do something a little different such that being unable to choose which to use is technically a restriction, even if you'd use them in order 99% of the time.

An example: Gunbreaker 1-2-3 goes 1. damage, 2. damage + selfheal/shield, 3. damage + resource generation. Normally you'd loop through them in order, but if for some reason you wanted to maximize your own durability (say the healer is dead and you need to live a little longer to let the DPS finish the boss) you might want the option to do 1-2-1-2-1-2,

Bard's generic attack and high damage proc can sort of justify being on different buttons because if you had to push two bombs away before returning to focus on the boss, you might want to Heavy Shot both bombs and then Straight Shot the boss instead of wasting your proc on a target whose health doesn't matter.