That wasn't meant to come across as showing someone up or even as an attack and I apologise if it did. I just wanted to make clear that even if there are cases when you could theoretically hold Holy for stunning at specific times it was never good practice, let alone mandatory.
Not in the past, when many things worked a bit differently and even less so now that you can simply faceroll through dungeons by burning everything down.

It's as you said: the only reason for a WHM to not use Holy is people needing healing that oGCDs won't cut or mechanics forcing them to move out of melee range.
Although I've seen more players simply eating an aoe in trash pulls to keep casting if it doesn't give a damage down, paralysis or silence.