Why is "them" more appropriate than "themselves" in the following sentence, even though what comes after with ("them") is the same as the subject ("They")?
They are taking all the system building blocks with them.
Why is "them" more appropriate than "themselves" in the following sentence, even though what comes after with ("them") is the same as the subject ("They")?
They are taking all the system building blocks with them.
We use 'themselves' when we are setting 'them' apart from anyone else.
For example:
They are moving house and transporting their furniture themselves.
In this example, 'themselves' is used to show that 'they' are actually taking on the task of moving their own furniture, rather than the common practice of employing someone else to do it for them. It is notable that they are doing it themselves, not anyone else.
That isn't what is going on in your example. 'They' are taking the building blocks. The expression to take something with oneself means that you take it along with you, for example when you leave somewhere it leaves with you. It adds detail about the manner in which you take it. It doesn't exist to suggest that you are taking it apart from anyone else doing so.
Many publishers try to have authors rephrase the sentence to avoid reflexive pronouns. Saguaro Books, my publishing house, also tries to avoid reflexive pronouns. Harbrace College Handbook Thirteenth edition.
As mentioned on this wonderful website, an object pronoun, and not a reflexive, should be used with "with" when it means "accompanied by."