Try retro-fitting a 50 ship Attack Fleet plus their Supply Ships to the latest Engines, Weapons and Shields, and as your Mineral Supplies drop you will quickly see the benifit of the Resource Converter.
In a PBW Game, retro-fitting my ships used ALL my Minerals, Organic & Radioactives that I had stored - 1.5M Resources in total. With the Help of the Resource Converter, I had just enough Minerals to complete the Retro-fit and the ships will be soon heading back to the front.