It depends on what you mean by "manufactured". In a sense, all sentences were created by human beings, including the ones that you just cited, but I suppose that you're using it to mean sentences which were created with the purpose of illustrating a grammar point, as opposed to sentences that were created spontaneously by native speakers in the context of natural speech or prose.
I think that whether or not using such "manufactured" sentences is desirable depends on the purpose for which the grammar book is used. If you use it with the intention of learning to produce good sentences, then I think natural sentences would be preferable, since they would better teach the most common contexts for particular grammatical structures. The reason for this is that while a sentence structure might be grammatically possible in a certain situation, it would be atypical to use that particular structure in that situation. Manufactured sentences are more likely to contain such atypical examples than sentences drawn from natural sources.
On the other hand, if all you want to do is learn to understand grammatical structures when you see them used, rather than use the structures in your own sentences, I see no problem with using manufactured sentences. Even if the examples are a bit unnatural, they can still help you understand the meaning of grammar points. There is no need to see natural sentences unless you intend to learn to produce sentences and not simply understand.
I think that whether or not using such "manufactured" sentences is desirable depends on the purpose for which the grammar book is used. If you use it with the intention of learning to produce good sentences, then I think natural sentences would be preferable, since they would better teach the most common contexts for particular grammatical structures. The reason for this is that while a sentence structure might be grammatically possible in a certain situation, it would be atypical to use that particular structure in that situation. Manufactured sentences are more likely to contain such atypical examples than sentences drawn from natural sources.
On the other hand, if all you want to do is learn to understand grammatical structures when you see them used, rather than use the structures in your own sentences, I see no problem with using manufactured sentences. Even if the examples are a bit unnatural, they can still help you understand the meaning of grammar points. There is no need to see natural sentences unless you intend to learn to produce sentences and not simply understand.