Firstly, there is some gameplay between cutscenes in that video as well. Not sure how much time it would add up to, but it definitely means the cutscenes themselves don't quite add up to 15 hours.
But more importantly, cutscenes are great! Assuming the story is good and they're worth watching (but are also skippable) then the more the better!
Oh, and as an aside, they spelt Aeris' name wrong!