The answer to this is generally yes. I have seen a very few books for which an index was not useful. When individuals are looking for a specific piece of information, they want to find it fast. A table of contents is generally not sufficient. An Index will generally increase your sales and text adoptions.
Many libraries require that a book have an index in order to be considered for purchase.
Since a good index is both an art and a science, it is more that an alphabetical list of topics with page numbers. Creating a good index requires an understanding of the subject metter and of the reader. Text analysis, patience, objectivity, speed, technical training, and experience all go into making a good index. A professional indexer can focus on the needs of both the novice reader and the more advanced and make all concepts instantly available to all readers. There are several ways to charge for an index-by the indexable page, by the entry, by the hour, or by the project.