1.) It's incredibly short. In general this should be a poor reason in favor of it, except I put it at the top of the list because the attention span of people today (mine included) is not what it was 20 years ago, let alone 91. Compare S&W with every grammar book you can find. Nothing comes close to having the same impact because nothing packs as much punch in as few pages. I've got several grammar books on my shelf, and over the years the one I remember the most, refer to the most and respect the most is the tiniest of them all. If you can't grasp its clarity and meaning, you are in the wrong profession.
2.) S&W don't force writers into rules. Their rules may be broken - permission is granted in the introduction and continues to the glossary. Why this is ignored I do not know. The only way to not know it's OK to break the rules is to not read the book, or pretend you didn't see dozens of sentences like this:
Style rules of this sort are, of course, somewhat a matter of individual preference, and even the established rules of grammar are open to challenge.
It is an old observation that the best writers sometimes disregard the rules of rhetoric.
This rule is difficult to apply.
This rule does not, of course, mean that the writer should entirely discard the passive voice, which is frequently convenient and sometimes necessary.
What's emphasized even more, though, is that every writer should learn the rules of grammar before breaking them.
This is the essence of the book:
When did clear, concise writing go out of style? What rules are outdated? I don't mean - what rules don't you like?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 sentences short or avoid all detail and treat subjects only in outline, but that every word tell.