Suppose I want to run algorithms on large arrays of 2D points while maximizing readability. I want to store the x and y coordinates using Python integers so I don't have to worry about overflow errors, but I expect that most of the time the numbers will be small and this is "just in case".
I claim that in this case, __slots__ is exactly the right thing to worry about.
It's hard for me to imagine that situation coming up, but yes, __slots__ does indeed have a purpose.
BTW, have you considered using the complex type to handle that for you? It's 2d and ints should be safe in float representation. If it overflows it'll crash nicely.
My point was not that __slots__ does nothing, but that there are more important things to worry about.