Any dry laid structure is only as good as the hearting used inside it. Hearting is the network of stones used to wedge, pin and bind the stones in a wall together properly. In just the same way a cemented wall is held together with a poorly mixed, or an improper mixture of, aggregate and bonding agent, will result in it not lasting long, so a dry stone wall 'laced' with round river stones, slippery pebbles, cubes of broken flagstone or crushable brick or friable rock fragments is not going to stand up for long either.
No, a good wall can only be as structural as the hearting is, and even if you don't skimp on the hearting, if you use incorrectly shaped stuff or place it badly your wall will fail.
We have recently found a plentiful supply of the best material ever to use inside a dry stone wall. It comes in a wonderful variety of suitably-sized sheared-off wedge-shaped chunks of stone, from a quarry floor where flat 3 inch slabs of stone are being squared-up by banker masons with hammers and chisels. The stuff we've been privileged to collect for free, though it looks like useless stuff to the boys at the quarry, is like gold dust to us. We use it, dare i say, like a dry mortar. It helps stones bind together, yet gives the wall capacity to move and not have stones slip or slump and so the wall yields and breaths and the stones with the hearting helps the wall shed moisture .Each carefully chosen sharp wedge shape fits perfectly between or under any combination of builder stones.
Good hearting material (like this or other similar sharp hard fragmented stone material) is the secret ingredient that makes dry stone walls great. To pin or fill any part of a wall with any thing less is 'heartless'.