the idea behind not doing a full, but only a half turn:
- avoid that a suface shares a vertice twice in the final PolySurface (red x dot)
- make sure that pull (which is part of most trim-Actions) has a easier job.
I remember some older CAD-exercises (not in Rhino) where Cylinders where represented with two 180-degree turns / surfaces.
And pull quite often does a messy job on special surfaces - see this topic:
(which never got much attention)