Because that is the more nuanced answer to "build or buy?" Of course you don't always want to build. But in borderline cases, err on the side of building, because there are often hidden advantages to building.
Are we clear now? Are you convinced I'm not advocating making your own electric power generation stations?
I think this was a terminology misunderstanding - I don't see it as "the answer". "The answer" in my book is specific for each particular situation and your guideline is important to take into account for each such situation. I'm splitting hair now, but only to illustrate the source of misunderstanding.
To advance the state of the art it would help if you created a list of cases where "build" produced unexpected advantage and a list of cases where it backfired. At least it would work better for me to calibrate properly.
EDIT: It just occured to me that you answer to my answer is what curried function is to a function result. Throw in lazy evaluation and the line gets blurry. :-)
