We find that in fact reciting Shemona Esreh of maariv is optional but rishonim and achronim describe it as obligatory. Well, if it has attained the standard of an obligation, then certainly chazaras hashatz should be required. It cannot be both obligatory and optional at the the same time, especially in the days before siddurim existed. But if despite it's categorization as obligatory as mincha and maariv there was never chazaras hashatz EVEN BEFORE there were siddurim, then certainly now when we DO have siddurim chazaras hashatz should be unnecessary even for mincha and shacharis.
David Goldman, USA
The Chidushei ha'Rashba (Berachos 27b, DH Tefilas) cites in the name of a Gaon that if somebody davens Ma'ariv once this automatically means that he has accepted it upon himself as obligatory. In his Teshuvos (1:183), the Rashba writes that according to this, Chazaras ha'Shatz is redundant for Ma'ariv. This is because the very essence of Chazaras ha'Shatz is to exempt someone who does not know how to pray it himself. However, if he has said it once, this must mean that he knows how to pray, while if he does not know how to pray himself this means that he never could have become obligated in Ma'ariv, so the Chazan is not allowed to say unnecessary Berachos to be Motzi him.
Kol Tuv,
Dovid Bloom