Are back-of-the-book indexes useful in electronic texts? I'm aware that people can search for anything they want in an electronic text and be taken there at the press of a button. But indexes don't merely point to terms located in the text. They also create relationships between terms and concepts through cross-referencing and subentries.
You essentially already answer your own question: indexes are more than just a reference to the words in a text. Apart from creating relationships an index can hide non-essential use of a word, and have definitions be different from normal use of a word. This requires interpretation by the author, something a search does not do.
I used LaTeX and its index generating capabilities when writing my thesis and the references to defining occurrences showed differently (italics) from the normal entries. Of course the software also helped by showing ranges of pages where a word was marked for indexing, which is also something a search engine seldom indicates (it might highlight all occurrences on a page, but that is not the same as knowing that a term is used in 10 consecutive pages).
Yes! Indexes are useful because they can contain entries pointing the reader to a page which discusses a topic without actually mentioning the topic's word.
E.G. A biography might talk about a person's difficult relationship with his wife. The book pages may say words like "relationship came under strain" and mention "Annie" repeatedly, but the index can add other words like "wife" and "break-up".