*Membership spots not really limited!
I think it's akin to referring to Aquaman as "Artie" or something. Sure, it's another name for Arthur... but you just wouldn't do it. Because it's not his name. And you wouldn't call the Human Torch "Jonathan Storm" either. Sure, it's a form of his name... but his name for all intents and purposes is Johnny.
These characters have shortened names for distinct reasons. And when you've got two characters with basically the same name, that distinction is even more important.
What about John Storm?