Tell me about the blue vectors, are they subject to change because they are not fully defined?

There are places where only splines will do and airfoils is a good example. I still use arcs as a first choice for most things because I can create the part with simple tools and a drawing. Wood ribs have splines for their shape but the wood isn't fully supported along its length if it's a stick rib so the accuracy is not going to be great in the finished part. If it's fabric covered there are a whole list of other deviations that happen. Still, you have to start somewhere.

The tangent arc version of the fork I drew would be indistinguishable from one defined by splines unless you could measure it very closely. Visually there would be no difference.