What's the point of outlining a focus, when one can't read an image
Web accessibility as a whole is a set of recommendations for ensuring that information on websites can easily be consumed by anyone and everyone. This is applies to every user: Mike, whose mouse no longer works; Jane, who is completely unable to operate mechanical input; Sarah, who is partially colour blind; to Steve, who cannot see at all.
The use cases for accessibility are so immense, that achieving Web Accessibility™ as a whole is an impossible task when time and budget are on the line.
Is picking a handful of accessibility features to focus on worthwhile? How is it fair to decide to make a website fully accessible to Mike, while telling Steve that his disability isn't important enough to focus on.
This practice of selective accessibility rarely stems from solid requirements or a detailed audit of content and user base. It often comes down to either things that can easily be accomplished without straying too far from the task at hand, or a simple "ooohh, this would be nice".
In theory, a little focus is better than no focus at all. In practice however, what is the benefit of forcing a highly visible border on a :focus element, when screen readers will have no way of reading an image without proper alt text? Why fight for infinitely scalable text, all the while ignoring contrast and excluding underlines on hyperlinks?
While I'm going slightly overboard with my thoughts on this selective accessibility, let's not forget the other, slightly different issue with accessibility. There is a widespread practice of forcefully removing default accessibility features implemented by browsers, all for the sake of making things "pretty". To save everyone time, that's something that can be addressed at a later date.
This half-assed approach to web accessibility not only accomplishes nothing on a global scale, but it often takes time away from enhancing the user experience.
Now there's a word: Experience. It has no distinct mention of either, but a good experience takes into account both usability and accessibility.
Whole-assing the user experience will have the added benefit of making sure that things are usable and accessible no matter what user type, device, or screen comes into play. Things just need to work.