In essence all models are originals designs.
All the separate meshes of a model can be considered lego pieces, because we can use them as such, but they are not designed as such.
For some pieces this makes a lot of sense to do, for instance a radar dish. It is an element that a lot of units share, so why not copy it instead of remodelling the whole thing?
But once you've started doing this it is very temping to save time by enhancing other units with reusable parts. For instance a length of metal pipe. You could say this is lazy but it can help to serve another important purpose; giving an entire unit-group or faction its own look, so to speak.
..although very original looking pieces can help with that too I suppose
