So does it have to be heavier to be better?

I think it depends on your definition of "better". My Audya and Pa4X are heavier than my S970, and in this case it translates into a more robust and firm feel of the case, and, certainly, the keybed. Maybe some of this is merely perceived as stronger, since the plastics today are supposed to be higher impact resistant. Psychological or not, I will forego light plastics and keybeds as long as I can.