Nobody is coding the sum of a list of integers for all the various integer types. People more likely use custom types anyway, and only have to create operations on those custom types ( except maybe for go language designers themselves). So there's always a suspicion that somehow there should be a way to code something clean using interfaces only.
People developing whole applications use custom types. People developing libraries meant for general use seem to prefer not limiting their stuff to working with their language's comparatively few built-in types (at least, that's how things seem to go in languages with generics -- Guava's BloomFilter should work with any type, even if that type was created well after Guava was written).
So that's why i'd like to see real-world problems that illustrate some concrete examples.