I've always felt that traditional indices in ebooks were a bit pointless. Anachronistically so. If print book makers could have made pages automatically turn and words on those pages to glow simply by saying a word aloud, then they'd have done so, and print indices would probably never have become a thing. And we wouldn't now be seeing people being forced by client dollars to try to simulate what a simple search engine can do with hypertext markup and millions of hardcoded links to and fro.
But I digress.
P.S. I've heard the "if you don't know what you need to search for" argument before, and I don't quite buy it. People who have no idea what they're looking for typically aren't looking for anything. And even if they were, manually wading through enormous, alphabetized, electronic indices is unlikely to focus their efforts very much.