[impendia]

Strongly agreed. I've taught university-level discrete math several times, and beginners to the subject need pegs to hang their hat on, something that serves as a goal and a signal that they're done. The Pigeonhole Principle is an ideal example of this.

Conversely, in research papers, or in conversations among math researchers (at least in my discipline) the Pigeonhole Principle is seldom mentioned by name. The idea is considered too "obvious" to need a name.