If you had merely mentioned "but this has a bug with respect to symlinks" and "but this regex is not correct for dot-newline and dot-dot-newline", your argument would be much more sound. But you didn't. So you picked an example that while at first seems like a good example to demonstrate recursion, is actually a known dangerous territory. This is my gripe. I guess the title of my rant isn't "stop reinventing the wheel", but "when you reinvent the wheel to teach, be sure you teach proper things!"
As for ...
To a newer programmer trying to understand directory (or any) recursions, it is a difficult enough task simply trying to comprehend the what, why, and how of it. To be burdoned by the many exceptions, catches, "look out for"'s, and so on.... would be discouraging, to say the least.
... that's exactly what programming is
all about! Especially with recursion, you must consider things like "will this ever terminate" and "am I looking at the right data" and "what can go wrong". Those cannot be overlooked, simply sweeping them under the rug.
The basics of recursion can be illustrated with something like factorial, which has easily defined end-points and recognizable computations. The flaws of recursion can be illustrated with something like the fibonacci calculator. But even these examples require at least a footnote to say "note that we aren't verifying that the number is an integer here", in my book anyway. Why do you think we have so many footnotes in the llama? I say one sweeping statement in the text, but in the footnote, I'm able to more carefully specify it so that I don't outright lie.
-- Randal L. Schwartz, Perl hacker