Do waste rocks have an utility?
Solution 1:
Yes, there is a tech called Wasterock Liquefication in engineering. It allows drones to convert waste rock to concrete. In my game it was very late (third to last tech in the row), but it pretty much instantly produced over 2000 concrete and cleared most of my dumping piles.
Solution 2:
Besides being able to make concrete from it using Wasterock Liquefication as described in Dulkan's answer, waste rock is used in building ramps and flattening uneven terrain.
In the "Green Planet" DLC, the Carbonate Processor consumes waste rock to improve the atmosphere.