> Repeated list and str concatenation both have quadratic O(N**2)
> performance, but people frequently build up strings with + 

join() isn't preferable in cases where it damages readability while
simultaneously providing zero or negative performance benefit, such as
when concatenating a few short strings, e.g. while adding a prefix to a
filename.

Although it's true that join() is automatically the safer option, and
especially when dealing with user supplied data, the net harm caused by
teaching rote and ceremony seems far less desirable compared to fixing a
trivial slowdown in a script, if that slowdown ever became apparent.