The tool your describing sounds like the extrude tool. which will extrude a profile along a path. Loft in maya will bridge 2+ profiles in order of selection. Yes, you are right. Sorry No new meshes are added to do this, it is not the same as using the planar opperation in NURBS (surfaces) which would create a new surface. All the planar tool does is create a polygon using existing verticies so no fusing is required. You would only get 'visible seams' if your output was NURBS since commands that fill the hole such as planar do create new geometry (thats just how NURBS work) however as long as your tesselation settings are correct the seam will not be visible at render time. I had converted the NURBS surface resulting from the extrude operation to a Polygon Mesh and the seam I was seeing was simply the extruded source shape - which as NURBS curve had a slightly diifferent outline than its polygon replacement. So I made the mistake to misinterpret that.