Author here, replying: A couple of reasons led to this: first, it was an experiment, and it was successful enough for this limited purpose that I thought it was worth sharing given the number of Movable Type users out there.
Second, the complexity of running the full packages with their limitations and lack of limitations (what you can't do and what you have to figure out from thousands of options to do) was daunting.
Third, my needs and the needs of many blogs-cum-lists is very particular, and this is entirely tailored.
In general, I'd rather choose a robust package from which I can carve out what I need. But this approach seemed better for this one case.