Why not? Indices are not pointers, so that's a poor argument by itself. Indices are offsets to pointers. If you can see any use for a negative offset off a pointer, that's your negative index use case. A nice example use is implementing IIR filters in a way that looks like the common mathematical form:

    y = a[0]*y[-1] + a[1]*y[-2] + b[0]*x[-1] + b[1]*x[-2]

Pointers are 64-bit on 64-bit machines, and so are the largest unsigned ints, hence the problem. On the other hand, you're probably searching at least 4-byte objects, so your actual list length isn't 64 bits but at most 62 bits even if you filled the memory space.