It doesn't matter if it keeps out Kryptonite radiation. HIS HEAD is still totally exposed. He used to have a lead-titanium alloy suit that he wore to keep Kryptonite out, and it covered his face. If the suit was really for protection, he'd cover his face. I don't care how badly he wants to be trusted and accepted, if the armor actually served a defensive purpose, it would cover his head.
My best guess is that his suit is invulnerable, but less so than him, and purely ceremonial. It
is Kryptonian in design after all. It's like a Scotsman with a kilt: it may offer a
little defense, but it's a lot more about heritage than protection.