My question: If that function were integrated in that surface WB in the future, would it appear while editing a BSpline in the Sketcher (and would do it so without a high computational cost / reasonable speed)?
Well, it depends on how flexible it is implemented. All the algorithmic stuff should stay in this surface module and then sketcher gui can link against this module or if a hard dependency is not desired use its Python interface.
I do see the merit of having it for arbitrary shapes. However I also see a specific tool ready to be used (it is already implemented), integrated in the Sketcher and apparently wanted by the users (useful). Of course yours is the last word, so if you want to see it excised, I will.
Since it's obviously already done it can stay there until the surface WB is ready. And it can still take a while until it's done.
But my actual point is that we should avoid to implement too specific functions in different modules which are also useful in a wider context. Because then we end up with dozens of slightly different functions and code duplications distributed all over the place.