That, plus the intimidation factor of the "mysterious, unknown hunter who always gets his prey." Much like Vader, Boba's hidden visage helped propagate his legend, notoriety and aura. Who's behind the helmet's visor? A man? An exotic alien? A droid?
It's not a bad selling point, really: not only is this guy the best in the known galaxy, but he looks very mysterious when he's doing it.