Gaieus,
Thank you for your very clear answer.
It was not so much as impatience, as perplexity. To me, it was those I had asked, "How do I copy an object in Sketchup, rather than move it?", and having over 50 people read the question, without getting a single answer.
You could avoid problems like this very easily: whenever someone clicks on a disabled button, display an explanation for why they cannot make that selection.