Since my last layout was a (12"= 12" scale) shortline, I would agree that a simple pile of dirt was most expedient as well as being less expensive (than ballast or crushed stone) to install and maintain. We had several such bumpers at various sidings.
From an engineering viewpoint, plain old dirt usually consolidates much better than piled course aggregate and isn't as likely to scatter. From a real world point of view, piled (unrestrained) course aggregates tend to scatter along their angle of repose when disrturbed with a love tap from a free rolling car, careless loco engineer, or rubber tired service vehicle. The cribbed bumper (with crushed stone or ballast fill) looks great and would do a better job at containing scatter, but would be succeptable to material theft in an isolated area.
I suppose it all gets back to the owner's choice in our wonderful world of model railroading.
Now retired, I am presently planning a shelf based shortline layout (in HO) that will be heavily based on actual operating experience. Will be modeling the WW-2 period on up through the early 1950's with mixed light steam and a couple of 44 ton diesel switchers.
Ted