First of all, I should make it clear that I believe it sounds better in all of the example sentence to omit the it. Whether or not it's technically grammatical to leave it in, it's probably preferable to omit it.
However, I don't believe that it's required to omit the it in sentences a or b—unlike sentences c and d, which I do believe must omit the it.
I should also mention that in the quoted reference of Practical English Usage by Michael Swan, Swan does not actually claim that the it must be omitted in any particular sentence; he simply claims that it can be omitted. Unless there is some additional text that has not been quoted, if I say that the it can remain, and can argue for it effectively, that's not necessarily going against the grammar book or saying that it's incorrect.
The way it's written may simply be meant as a way of emphasizing contrast rather than prohibiting grammar:
Emotions can be used in place of calmer statements.
That's fantastic! NOT
I feel positive about it.
Here, neither sentence is ungrammatical. It's simply meant that the same thing can be expressed differently. If I'm being fair, I grant this isn't likely. But the point is that he's not being explicit about what he means by the styling—we're making an assumption. On the other hand, people do make mistakes at times.
(I will note, however, that in his second example sentence, which uses they, that they cannot be used as a dummy subject. Only it and there can be used as dummy subjects. I believe it's the use of the dummy subject that allows for the presence of it in the first sentence. But, in the second sentence, I think it actually is the case that they must be omitted.)
As for the dummy subject, consider the following two sentences:
It is raining.
It is nice in Florida this time of year.
In neither of these sentences does it refer to anything specific, as would a normal pronoun—or would it in more conventional syntax. The reason it exists is because it is acting as a kind of generic grammatical placeholder rather than as a reference to something specific.
From Cambridge Dictionary:
English clauses which are not imperatives must have a subject. Sometimes we need to use a ‘dummy’ or ‘empty’ or ‘artificial’ subject when there is no subject attached to the verb, and where the real subject is somewhere else in the clause. It and there are the two dummy subjects used in English:
It’s always interesting to find out about your family history.
To find out about your family history is always interesting. (The real subject – the thing that is interesting – is ‘to find out about your family history’.)
There are five Dutch people in our village. (The real subject is the Dutch people – they are in the village.)
We often use it as a dummy subject with adjectives and their complements:
It’s important to wear a helmet whenever you do any dangerous sport.
Wearing a helmet when you do any dangerous sport
Is important to wear a helmet … (The real subject is ‘wearing a helmet when you do any dangerous sport’ – that is what is important.)
It’s useful to write down your passport number somewhere, in case you lose it.
The Cambridge Dictionary entry discusses there in more detail too, but it's not as specifically relevant.
This sentence is grammatical:
You're taller than described.
This also happens to be what I think is the most natural version of the sentence, and what most people would use.
But let's look at a rephrased version of sentence a:
It is described that you are tall.
This makes use of the dummy subject it. It is a pronoun that doesn't refer to anything specific.
This serves the same function as:
It is said that a rolling stone gathers no moss.
The fact that we don't know anything specific about the who or when or how in relation to the saying doesn't matter. It's still a valid sentence. Similarly, so is the it is described sentence—even though we also don't know anything about the who or when or how in relation to the description.
If we can use that sentence, where the use of it makes sense, then it seems to me we should also be able to use the original sentence:
You're taller than it is described.
Additionally, the following would also make sense, if we thought of the sentence as elliptical:
You're taller than it is described [you are].
A similar analysis could be applied to sentence b:
He worries more than necessary.
It is necessary he worry.
He worries more than it is necessary.
He worries more than it is necessary [for him to worry].
While neither a nor b are the most natural sounding of sentences, and I would rephrase them to the simpler versions I provided, I do not believe that either is actually syntactically in error. Nor do I believe that Swan specifically says that it must be omitted, merely that it can be omitted. (And in my opinion probably should be omitted in most cases.)
As for sentences c and d, I'm not nearly as convinced that they can ever be grammatical. Or that, if they can, the amount of editing to make them so would be worth it.
The construction of sentences c and d is different.
With sentences a and b, I can make positive use of a dummy it:
✔ It is described that you are tall.
✔ It is necessary he worry.
But I can't think of a way of doing the same with c or d:
✘ It would otherwise have been the case.
✘ It would otherwise have been possible.
As purely standalone sentences, the would otherwise doesn't attach to anything. It's not an affirmative statement about an existing situation or condition.
Both of these make sense:
✔ It is the case that two plus two is four.
✔ It is possible that it will rain tomorrow.
In those, the meaning is clear within each sentence itself.
The only way that the other sentences could possibly make sense would be if they came within a broader context:
✔ Because she ate within the last week, it wasn't the case that she starved to death. It would otherwise have been the case.
Now, the second sentence makes sense. But only because it's been given proper context.
So, considering everything so far, we could provide additional context to sentence c and state a different initial form:
✔ Because Germany had a weak leader, it wasn't the case that it adopted a strong currency. It would otherwise have been the case.
But note the lengths I've had to go to in order to get it to this initial point—and it only works because I've added another sentence with some additional context and emphasis.
I can think of no way that I can easily—or usefully—make this correct:
✘ Germany adopted a much weaker currency than it would otherwise have been the case.
Possibly I could come up with a two-sentence version that would allow for it to be grammatical with the use of it. But in doing so it would become at least twice as long—and most likely add extraneous information.
Even if it were possible (and I can't think of a method at the moment), it's far more simplistic and succinct to just drop the it in this case.