^It's not so much about identity as it is about persona, if that makes any sense. Like with Dredd, there's no mystery as to what's under the helmet (Dredd looks like Fargo and Fargo's face is known), that's not the point. The point is that to all intents and purposes the helmet *is* his face.
Again, I know it's not 100% rational but before AotC Fett was a version of the mysterious gunslinger trope, like "The Man With No Name". In that case, it doesn't matter that he really does have a name, you don't want to know it. It's irrelevant and it would de mystify the character.
From an in-universe perspective there's good reason why Fett might never take the helmet off except in private: thanks to the Clone Wars the face of Jango Fett is very recognisable and he'd just as soon not be taken as some old clone trooper of deserter. Or maybe it's deeper and he'd rather not look at his reflection and see his dead father staring back at him.
Just to be clear, I'm not dead-set against him being seen without the helmet on, but I feel from a mythic storytelling perspective, once he starts wearing it, the audience should not see him take it off again.
