Even if people often cohabite with other gender's sex nowadays, with just being friends, they never raise children together - let alone one of them who consider them as his parents.
Denzel, IMHO, is clearly there to cement this side of their relationship - that way, no one can say 'oh but they're only friends'. If you watch a movie with parents trying to save/protect their kid like they do, having a chat together like they do [my bf's WTF's face when he saw the Turks in the room during that scene was hilarious, he even asked me 'wtf are they doing there', as the convo between Cloud and Tifa felt very intimate] - there's no way you can say 'they're only friends'.
Because it shows Cloud hurting, being afraid of losing his family, of finding them again - ACC definitely puts that into perspective. The ending hammers Zerith, in case no one suspected anything regarding Aerith and Zack
Which also contributes for the Clot and Zerith canon pairings, as opposed to Clerith [that simply doesn't fit in the narrative of the movie, and goes against what's being shown with Cloud finding his own Promise Land with Tifa and the kids and the very clearly Zerith pics at the end].
If anyone had doubts with AC, I think that ACC cleared them - and that was also SE's goal. Because they gave depth and prevented people to imagine that Cloud was going in lalaland with Aerith in a flower field at the end - I also think that it's why Aerith disappeared from the ending - and suddenly, it all makes sense.