I came across this blog post in the Chronicle of Higher Education (http://chronicle.com/section/Home/5)
In it, the author describes her steps in indexing her own book and how she has come to the conclusion that it is worth the cost to an author to pay for the services of a professional indexer.
The author also sums up why she feels indexes are still necessary for documents electronic or otherwise:
Despite the fact that books are increasingly becoming searchable in their electronic formats, the metadata that’s provided by a good index can have a great influence over how the book is discovered, and how it’s used. A good index is more than just an alphabetical list of all the text’s proper nouns and their locations; it’s a way of thinking about the ideas within the text that can guide a reader to the sections they most need to consult.