I think the argument is less "noone but power users would need this" and more "only power users will know how to apply 'Invert Selection' to get the behaviour they want".
ie there's an inductive leap between "I want to delete all files except this one" and "I can select that one file and then invert selection before deleting". At minimum it requires you to know what "invert selection" means. Ordinary users may take a different, more fiddly, path.
I don't know if that's the case here. My intuition says it is. However, in my optimistic heart I hope Microsoft did a bunch of usability testing and determined that ordinary users do grasp an "invert selection" concept more often than not.
That's how design for mass-use user interfaces works, right? By evidence-driven testing, not by what the programmer/designer thinks makes sense? Clings to idealistic belief.
Select all, then deselect (ctrl-click) the one item you want to exclude. That's the exact same amount of work, just with the inversion step at the beginning instead of the end.
People who don't know about menus shouldn't really be mass-deleting things yet. Alternatively, add a button for "Help! -The Computer lost all my stuff".
A brief Google search confirms that "lost my files" wasn't solved in 1995. I'm pretty sure those aren't all Mac users. Even if 90% are due to other issues, there are many people who would like their data back. I believe many are due to the ease with which a user can delete files compared to the difficulty they experience when restoring them.
The dangerous button is much easier to find than the fix-it button. One is right there, with your files. The other is someplace else, related by metaphor alone. Once you have selected the recycle bin you then have to search through your very long deleted file list, select the file you want, select another button to restore it (which appears in the same place as a different button existed before selecting the file), and then navigate back to the place you were expecting the file to be. We're talking about users who can't figure out a menu, but they can perform this multi-step task. I can easily see a new computer user screwing this up.
Next time you look at the context menu of a removable drive, see that "Format" is right there with the eject menu item. One should be used every time you use the drive, the other very rarely and only when you're sure. The latter should be removed to a sub-menu under "advanced", or similar.
Myself, I'd make it harder to delete things and a lot easier to find them again.
