Assuming you mean a back-of-the-book alphabetical index: my short answer is
no. Here’s why.
Many (perhaps most) chapters in most books are incompletely or poorly
indexed*, if at all. Therefore IMO having no index Is better than having a
bad one - and an index isn’t really necessary in a PDF, though it’s highly
desirable in a printed book and can be useful in PDF if good use has been
made of synonyms. Also, unless a book index has been reviewed and edited by
one person, it’s usually very inconsistent and/or includes many irrelevant
entries.
*And some still include entries for old terminology, going back as far as
the OOo
incarnation of the book.
With our rapid development times and lack of people (especially people with
indexing skills), I think it’s impractical. OTOH, if someone has the time
to do it without delaying release of a book, it’s definitely a
nice-to-have. BTW, I tried to recruit indexers, but they all fled from the
requirement to use OOo/LO embedded index entries.
[My opinion, based on over 15 years volunteering with OOo and LO
documentation.]
Jean