Polymer mil ammo cases have advantages other than weight reduction. During tests, poly cases had a far longer "cook off" time than brass cased ammo, and this was due to the poly blend acting like an insulator were as brass transfers heat to the powder/primer more rapidly. Poly cases tend to be more "slick" than brass thus improving loading and extraction when dirty. Under military environmental conditions, Poly does not corrode as brass does, and thus can be more reliable.
ETA: During drop and damage tests, Poly cases are far less likely to misshapen and often return to their original formed shape and function, where as brass often deforms and causes FTF issues.
Disadvantages: Poly does not extract heat from the chamber like an expended brass case will. Cost is one factor in the "initial" process, but as production ramps up for mil demand, costs does come down considerably.