Page numbers changed by third party without rerunning makeindex
Amazingly often a third party changes the page numbers without rerunning
makeindex. (Yes, it happened also to my 1993 book.)
It is much nicer to have an index that adapts itself to all
changes but the adding/deleting of an \index{entry} command.
Only after such adding and/or deleting do we want to run makeindex.
Index contains no destination for hyperlink
The theindex environment lacks a destination for hyperlinks.
That is a nuisance when one wants to add the index to
the bookmarks or to the table of contents in a pdf file.