Depends on the application obviously. A lot of social groups would likely be meeting once a week. It's just a simple example though of how the step function could be useful.
For your example, you would just need a step with some kind of range. So you could say step=7, range=5 to allow them to pick from the first 5 days of every 7 (or to get trick step=7 range=-2 to eliminate the last 2 days). I'd hazard a guess they thought about this kind of stuff, but realised it would be complicated pretty quickly.
Most applications are enterprise apps, the vast majority of apps in the world today are enterprise apps. That work Mon-Fri, so want to restrict date pickers to Mon-Fri.
And it doesn't have range, does it? So the infinitely more useful range hasn't been specified, but the almost useless step has.
For your example, you would just need a step with some kind of range. So you could say step=7, range=5 to allow them to pick from the first 5 days of every 7 (or to get trick step=7 range=-2 to eliminate the last 2 days). I'd hazard a guess they thought about this kind of stuff, but realised it would be complicated pretty quickly.