There are many factors as to why, or not, to use filler material. First off, it would be underestimation to assume that all "filler" material is bad. When used properly it allows for more options with core dynamics. Cores can be tweaked with different densities and sizes if the fill material weighs less than cover material.
There are probably advantages on both sides of the equation. If more cover material is on every ball it should be more durable, but there will be a certain amount of trade off because perhaps the company can only produce 14, 15, and 16 lb. balls, shrinking the marketplace for such ball.
There are no absolutes. What works well for Bowler A may not be best for Bowler B. It's another "arrow in your quiver" to have a variety of equipment.