My problem is one of productivity. There's already a standard library, and if it's a language I've been using for a while, I probably remember most of it. I can pretty much go straight from my thought of what I want done to typing it out, much like typing English. If you force a 'cache miss' and force me out of my head and into a documentation search, well, that's going to have a significant effect on my performance. If the function is well-named, it has less of a cost in reading the code, but there's still a cost, because what if it's slightly different? I have a pretty good feel for the gotchas in much of the standard library of the languages I use. I have to stop and check what the gotchas of your function are.

Yes, at some point the complexity cost of gluing together the standard library functions to do something becomes greater than the lookup cost of finding a function that does what I want; but I am saying that adding more functions is not costless.