At the risk of falling down an analogy rabbit hole, I'll be upset if the HVAC guy assumes that air will flow freely throughout the house and has no understanding of walls, or if the drywall guy blindly screws into my air ducts. No abstraction is perfect; some knowledge of the other layers is necessary to do a proper job. Unfortunately, in software, it seems like our abstractions are particularly leaky, and knowledge of other layers is frequently necessary to do a proper job. In house building, issues are usually contained by physical proximity, whereas the same is obviously not true in software, particularly networked software.