of a book I'm reading
Which one? What's your view on it?

writing assignment-less code is as important in functional programming as it is not to use goto in structured programming
Yes, that's what I've come across as well. But what I'm not seeing is a good explanation for c++ specifically as to why. Much of what I've seen is 'this is how the xyx functional language requires it to be done'. Well for a 'pure functional' language that's fine. You learn how things are done in specific languages. For c++, I've seen this is how you can implement 'functional' programming. However what benefits does this style of programming bring to c++??????? At the moment, like you, all I'm seeing is complexity and potential performance issues.