Distributed DC power is actually used in some new construction. It's driven less by the efficiency of the transformer than by other logistics. Here in California, at least, we have a law (known as Title 24) which requires some fairly sophisticated controls of lighting, as well as fairly low power consumption per square foot. The control requirements include compensating for daylight by automatically dimming lights, occupancy sensing, and brownout usage reductions. This means that the controllers are fairly sophisticated and expensive, and that LED light fixtures are sometimes preferred in new construction. As a result, some systems are being sold with the controllers that output dimmed DC which directly powers the LED fixtures.

DC distribution reduces the number of wires that have to go to each fixture (DC+ and DC- instead of AC hot, neutral, and a separate control line) and saves some money on electronics. As far as I know though, there is still at least one controller per room. I'm not aware of any systems that distribute DC all throughout a building. I imagine this is because as lengths and currents increase, the advantages of high voltage in reducing wire size become more significant.