> The only time you should actually override __iadd__ is for mutable objects

What would the rationale be for such a rule?

the docs don't mention anything like that https://docs.python.org/2/library/operator.html and I've never heard it before. In Python, neither integers nor strings are mutable and both support the "x += y" syntax.

The __iadd__ method works the same for both mutable and immutable objects AFAIK