Hi Danny, well in addition to moving control points there are some functions like Flow where the particular control point structure of the surface has an impact... So replacing surfaces in extrude could then have a side effect of some unwanted changes in behavior when then using flow on that surface afterwards.

I actually do have a similar type of surface replacement mechanism in Revolve currently, where if you have a line perpendicular to the revolve axis it gets replaced by a trimmed plane just like that, and it has indeed gotten a couple of people confused when using Flow with those particular surfaces...

On the plus side a simple plane surface does tend to work better for intersections calculations.

- Michael