Ease of learning is often decoupled from ease of efficient use, sometimes even inversely related. 'Usable' is not one thing for everybody. So: a skeuomorph is

Ease of learning is often decoupled from ease of efficient use, sometimes even inversely related. 'Usable' is not one thing for everybody. So: a skeuomorph is obviously not a problem if it does not cause gratuitous errors or slowdowns, and it obviously is if it does. And in any case design also concerns matters of taste, on which opinions will naturally differ.