I think X might be anatomically correct, because Dr. Light was a bit of a pervert eccentric. In my experience, most men think male sex organs are good things to have, and would definitely want their "son" to have them. Zero, Sigma, etc., probably don't, because Wily and Cain didn't have the same kind of emotional investment in their creations, I think (not that they didn't have strong feelings, just not the same kind as Light).
You have a very, very good point, I have to say. I was going on MHX, and having that Light made X in the most practical way because he was dying, and didn't have much time left and in my eyes didn't give him a sex organ, and more in my preference, no hairdo either with the helmet being part of his head altogether. It was also a preference that X didn't have any organs either since I felt that's what made him different.
However, when you pointed out that Light called X HIS hope, rather then the world's in the Japanese version (since that disassociated him from X then), it makes sense that Light would want X to be more human then humans themselves and not just for the yaoi fangirls :D
That, and taking the Star Trek view, Data the android has human aspects/organs, and learns about human emotion, I can see why Light would want the same thing for X. If he's to help humans, why not have him like them to make the whole thing a bit easier? That and for Zero, if you really need to get at his noggin, lift the 'lid' of his skull (like Data) to get at it. Same with X, if you like him with hair too. I like Zero having a full head of hair too, but I like him also with the helmet on 99.9% of the time, so the style never really crossed my mind. I like the Hibiki style, but I'm not keen on the eyebrows myself.
Zero, Sigma, etc., probably don't, because Wily and Cain didn't have the same kind of emotional investment in their creations, I think (not that they didn't have strong feelings, just not the same kind as Light).
I see that too - Wily wanted a war machine, and look at Forte. His helmet is a cobra shape, so it's an associated shape to cause fear, so why not a horned helmet, and the long hair? It's distinctive, and in some cases, cape-like, so Zero would be a demon (well, to me anyway *has a big longwinded idea to X and Zero's designs*). I think with Cain, Reploids could choose to make themselves up if they wanted, like Blunderbus' idea.
raging cases of helmet hair
Not necessarily, it could be resilient fiber with a 'memory' for its original state. It could bounce back as quickly as you want.
On that, would Reploids even sweat to cause helmet hair anyway?