So, the button merging feature... did I get this right?

[Cynric]

You have two enemies, one is at one HP, the other is full. You are at the end of your combo. You could use the combo finisher on the damaged mob, wasting potency or you could reset by going to the combo starter which would allow you to start attacking the second mob with the higher damage combo 2 button. DPS gained.

This is a niche situation, but there are drawbacks to forced combos. They are less interesting to play in my opinion and they take away optimization options.

It makes more sense in this situation to swap to the target with higher health with your combo finisher.

Like roughly say we are a samurai,

You can get 380 + 200 = 580 from Gekko into Hakaze (+15 kenki)
or you can get.. 200 + 280 = 480 for Hakaze into Jinpu/Shifu (+10 kenki)

I don't see why you would ever divert down to a first step combo, its just a direct potency loss. You're losing 5 kenki and 100 potency.
This isn't even mentioning that you'd lose a sen.

If this situation were an auto combo you would just finish the auto combo, it would be silly to go back to hakaze when you have gekko/kasha ready.

That's assuming a two target situation as well, if it were any higher we'd just use aoe combo, and that's really simple as well, mangetsu into fuga is going to just be better than fuga into fuga.

2(90 x 3(n) ) vs 90x3(n) + 120 x3(n)
and 10 kenki vs 15 kenki.

I'm sure we could look at other jobs and see similar situations.

I'm not exactly for auto combos 100% but its hard to see how restarting at first step combos is beneficial.

I think it's great they've given it as an optional choice, best of both worlds.