Not sure, but like the native Rhino command, rs.AddCutPlane takes the CPlane of the active viewport as a reference, so maybe the wrong viewport is active at the time the script is launched (depending on where/how you pick your points) ?
The Help is a bit confusing though, looking at it again, I can’t figure out if it is using the active CPlane or World…
Edit -
OK, I did a little investigation and it looks like the rs.AddCutPlane() method may be unreliable if the objects to be cut are all coplanar and on the same plane. It uses the bounding box of the objects to determine the depth of the plane and since the box is flat, in Z it may be getting confused - it adds a plane but the plane does not cut through the selected objects (check this by not deleting the cut planes at the end of the script). I don’t have the time to investigate this further today though.
One other thing briefly, you do not need to delete the points 0 and 1 as they are not point objects in the document anyway.
Edit 2:
I have tracked down a bug in the rs.AddCutPlane() method which looks like it is causing your problem.