That's really difficult to answer (the question in the OP), because it's dependent upon what level student you might be referring to. Our association has a series of one-steps which are required in testing, even into advanced dan ranks. Personally, considering them as practical defense exercises, I think they pretty much are useless. However, especially for colored belt students, one-steps teach range, speed, fluidity, etc. AS you noted, the ubiquitous stepping through is probably designed to allow the defender time to react, but is fairly unrealistic. One-steps do create a very good foundation for defense, I think. We try to adjust the attack and defense as students advance, to make them more real. But we still have those mandated one-steps which are a prerequisite for testing at every rank. We always make a point when teaching the mandatory techniques "Never, ever, do this is real life." or "This one will work and is pretty good." etc.
Having said all that, I like teaching one-steps and the students usually enjoy learning them. We do mix things up a little, to keep it interesting. So, for instance, we might form 5 or 6 students in a circle, with one in the middle. Less experienced students must make the stepping through attack starting on a designated foot, and the outside circle attacks, in order, only on a command from the instructor. As they advance, we change up the attacks, then allow random attacks, etc. Circle drill, everyone loves it. But it's really just one-steps.