Depends what you mean by "Giant Polyreme". If you mean any ship bigger than a pentere, then you are mistaken. During the Hellenestic era, ships such as hexereis, heptereis, etc, became fairly common and were definately used in battle, although the pentere remained the main line-of-battle ship. To take an example, in Memnon's history of Herakleia, we hear that during the war between Ptolemaios Keraunos and Antigonos Gonatas, the Herakleian fleet (which fought on Keraunos' side) was made up of "Hexereis, pentereis, un-decked vessels and an octere". The latter, probably the flagship, had (according to Memnon) 1600 oarsmen (making her one hundred oars in length--that's alot!) and 1200 soldiers and mariners on the decks, and two helmsmen; Memnon states that this ship was the most effective during the naval battle. He may be exagerating the ship's complement, but I don't see why we should refuse to believe that the polis of Herakleia Pontika had her very own "super-ship", which actively participated in at least one naval battle, and was very much intended for war as well as for show.