Omit needless words.
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that every word tell.
I enjoy this paragraph a lot, and take pleasure in identifying and removing my own unnecessary words. I do, however, always find myself pouring over this particular paragraph, wondering to myself: “is adding ‘a machine no unnecessary parts’ really necessary? Should he have cut ‘treat his subjects only in outline’?”.
The book has gone out of favour in recent years and been described as consisting of “uninformed bossiness” and being an “aging zombie of a book”.
The book is certainly prescriptivist, but that is the nature of style guides. They pick a particular rule and apply it consistently across all their content. In cases where there is no right or wrong answer (such as the Oxford comma), they ensure consistency. For magazines and newspapers, the only wrong rule is not having a rule.
With a guide on general English usage, it’s a bit more tricky. And the tone of The Elements of Style has certainly dated. Nevertheless, it’s wrong to confuse the tone with the content. Even though William Strunk wasn’t entirely sure what the passive was, and had rather dated views on split infinitives, there are many rules that remain true. While it might not be right to demand that everyone omit needless words it’s certainly right to strive for it in your own prose.