It is my hunch that there is too much control with this curve on surface tool. Since I am not a programmer it may be difficult to explain but it is my belief that this tool is likely un-usable because it gives users control over length at internal edit points at each edit point as well as control on it’s end points. Basically the tool is too complicated and any small edit pushes the spline un controllable.
I don’t have a contact at SW but if I did I would suggest to their product line manager to to their programmers to consider removing some of the control over that specific curve tool… take this bezier curve control with the internal edit points then I believe the tool to be much more usable.