This is a good point; however, it rests on the assumption that Strunk and White managed to accurately describe what they are doing. But actually they failed at this.
You are stating this as a binary fact (either they did, or they did not, accurately describe what they are doing). But surely what is more relevant is not a binary fact, but rather a matter of degree. Two questions are important:
1) What portion of their own advice did they not follow?
Suppose you find ten things that Strunk and White didn’t do that they said you should do. That amounts to a page of errata, which many books have (and which maybe all books should have). If we threw out every book that had (or deserved) a page of errata, then we would probably empty the libraries.
2) To what extent did they not follow it?
Question number (2) is interesting because it’s not always a question that can easily be answered by looking at their writing. Here’s what I mean. Suppose that I write an essay that is 100% passive constructions. Then I remember the advice to avoid passive constructions if possible. So I go through my essay, find a lot of passives that would be strengthened by making then active, and bring my essay down to 80% passive constructions.
Now, somebody looking at my essay will see that it is 80% passives and he might be tempted to conclude that I didn’t follow the advice to avoid passives. And he would be wrong.
You are stating this as a binary fact (either they did, or they did not, accurately describe what they are doing). But surely what is more relevant is not a binary fact, but rather a matter of degree. Two questions are important:
1) What portion of their own advice did they not follow?
Suppose you find ten things that Strunk and White didn’t do that they said you should do. That amounts to a page of errata, which many books have (and which maybe all books should have). If we threw out every book that had (or deserved) a page of errata, then we would probably empty the libraries.
2) To what extent did they not follow it?
Question number (2) is interesting because it’s not always a question that can easily be answered by looking at their writing. Here’s what I mean. Suppose that I write an essay that is 100% passive constructions. Then I remember the advice to avoid passive constructions if possible. So I go through my essay, find a lot of passives that would be strengthened by making then active, and bring my essay down to 80% passive constructions.
Now, somebody looking at my essay will see that it is 80% passives and he might be tempted to conclude that I didn’t follow the advice to avoid passives. And he would be wrong.