[Silhouette]

I am simply suggesting that religiously hiding implementation of simple data structures behind formal interfaces may be excessive.

You seem to be assuming a certain style of programming or type of programming language here, along the lines of mainstream OOP languages. There are other options, from low level languages like C to languages where algebraic data types and pattern matching are bread and butter, that do not attempt to conceal true representations of simple data structures in that way. The sky does not fall! :-)

Sometimes it is useful to be able to drop down to that level if the provided set of standard functions to work with that data structure does not meet your needs. You might want to support some new access pattern that is not offered in a convenient form by the tools available in the provided interface, for example.

Sometimes you might want to know, say, which order the axes go in for some multi-dimensional data structure, because writing your algorithms to match can have a profound effect on real world performance. And in this sort of situation, the advantages of being explicit about that can far outweigh the theoretical benefits of being able to replace the internal representation transparently, which is something you are extremely unlikely to be doing in an established system where this kind of issue arises anyway.