The reason that manufacturers use the "filler" is exactly as Greg7 says. To make a lower RG ball, weight must be added to the inner core. The only way to do this is to replace some of the coverstock thickness with a material that is less dense. If you made the inner core more dense without an outer core, you would increase the gross weight at the same time. Also, the USBC has a restriction on the density of materials to be used in the inner cores, so it is difficult to create the necessary mass in a small volume core.
Contrary to some opinions, ball manufacturers do not use the filler material to lower the cost of the product. It is a tool to be able to offer a wider range of RG's and differentials to the consumer.
--------------------
Brian Pursel
Director of Sales
Track International
Evolutionary. Revolutionary.