I proposed, based on Petrie's observations of an impossible conventional feedrate, that the pressure was provided by the screw and nut method. With only the screw and nut and no pounding action, we will advance the tool as far as the workface and go no further. I heard this argument on DejaNews, too. In a perfect world, all machined surfaces would be free of tool marks. If you were observing holes ultrasonically drilled into expensive optical components for NASA or the ESA, there would be no striations.
Most ultrasonic machines plunge directly into the material without rotating the tool.
Most EDM machines do the same. There are exceptions to both. There are also exceptions to the requirements for smooth striation-free machined objects. Take grinding, for instance. A grinding wheel can become clogged with the material it is removing which slows down the grinding process.