The way I've heard it phrased best is the way (I think) Allen Ward put it. If a component is high cost, tightly integrated with the rest of the system, and dif

The way I've heard it phrased best is the way (I think) Allen Ward put it. If a component is high cost, tightly integrated with the rest of the system, and difficult to design, then you need to be an expert on it in order to evaluate suppliers and make a good purchasing decision. To become an expert, you have to build it for a while yourself. And once you have built it for a while and become an expert, you might as well keep that up, unless there's a strong economic incentive not to.