I honestly didn't expect them to have the balls to throw us an actual fight with Sephiroth. I thought we'd merely fight a copy of him that would turn out to simply be an illusion or revealed to be an offshoot of Jenova.
I never expected Sephiroth (or some variation) to literally decide to step in and in essence, get his hands dirty. It's was a left field occurrence. How they explain it and utilize it remains to be seen, but it was enjoyable, interesting and I hope it's reason fits the overall plot of the story in the end.
Honestly, the fight with Sephiroth in Chapter 18 reminds me of how 3 years later in KH2 we discovered that we actually fought with Xemnas himself in KH1 Final Mix. That bizarre, mysterious and simply baffling encounter with the Unknown enemy in Hollow Bastion was actually an encounter with who would go on to be, the final boss of the next game.
I don't know how it'll make sense within the specific context here, but it's an example of how a seemingly bizarre and incongruous boss fight actually laid the groundwork for the future and ultimately made sense in retrospect.
