Glass composition was experimentally determined so that it has needed properties.
From that statement, I would conclude that without the metal ions, glass does not have certain needed properties, and with metal ions, it does. My question is: what are those needed properties that are effected by the various ions?
Needed properties are temperature at which glass becomes soft, coefficient of thermal expansion, chemical resistance and so on. But I strongly doubt it is possible to assign single property to a single ion.