From the discussion in https://mail.python.org/pipermail/python-ideas/2019-April/056416.html, Guido said:

> Some use cases for rindex() on strings that I found in a large codebase
here include searching a pathname for the final slash, a list of
comma-separated items for the last comma, a fully-qualified module name for
the last period, and some ad-hoc parsing of other things. The "last
separator" use cases are the most common and here rindex() sounds very
useful.

I guess that's strong enough.