Sure, that is an absolutely sensible design decision. But for other cases completeness is vital. Consider for instance the trivial case of the map function over lists (constructed by :: and []), often abbreviated as * :

  f * [ ] = [ ]

  f * a::as = (f a)::(f * as)

Even if that is a nearly trivial case, it is easy to see that one could forget the first case, as all the interesting bits happen elsewhere. Now imagine, I would want to have a lazy reversal of lists, that I store as an alternative until I need to deconstruct it. I write that x~xs, meaning "x added to the end of xs".

Now I have to expand my map function as follows (because map does not care about ordering):

  f * a~as = (f a) ~ (f * as)

If I miss that case (or method on Julia's case) my extension of the datatype is plain wrong.