...because zebra's are not a universally useful modelling tool to programmers like references are. Thus, the absence of a reference, ie nil, also becomes a useful, commonly used modeling tool. If we all wrote software using african wildlife metaphor, 'non-zebra' might then be just as useful.
> ...because zebra's are not a universally useful modelling tool to programmers like references are.
References are not the zebras. Nil-references are.
We might as well say that the underlying number that the reference are represented by are useful modelling tools, in some circumstances. That doesn't mean that you want pointer arithmetic for references all the time.
non-zebra numbers[1] are also very useful. But why have a "non-zebra number" when I can just have plain numbers?
[1] A "number" which is either a number, or a zebra