We need such thing in order to add a larger variety of shapes avialable to be made for fuselages.
For example, it'd be helpful for making things such as Bezier curves, which can only be "partially" replicated with a lot of panelling.
Example of such fuselage shape:
With CSE, the part count would decrease dramatically, as it'd require much less panelling, if even at all. This would also benefit mobile players, because their devices wouldn't handle very detailed crafts very well, due to the big number of parts.
Not only that, but it'd make the job easier for a lot of players that make SSTOs or planes.
Overall, I think that such feature should make it into the game.
Also, I think this feature would blend well with other planned sugestions (like fuselage slice and Truly Spherical Tanks, shapes)