Thanks for your review @Campbell Barton (campbellbarton) . Good catch, I noticed that it's essentially the same for loose edges.

I've updated the patch to reflect both cases and attached a blend with all possible scenarios I can think of (mesh only, point cloud, mesh with loose edges and vertices etc.).

The exporter is now able to export all components of the geometry. However, I had to re-write the code to maintain the vertex indices for reference. Positive side effect is that in case the mesh has no uv's, the exporter now only writes single vertices an their attributes to the file, instead of writing all vertices per face. In my tests all my scenarios are working and the geometry came back to blender as expected. The importer seems to handle all that cases very great, although we could add removing possible doubles at the end, but I'm not sure whether that's good practice switching to edit mode and so on...

What do you think?

Edit: There are only 3 components per vertex color, right?

Thanks for your review Campbell, it really helped me pushing the patch forward. I rolled back all unnecessary changes as requested and hope that this allows a better review.

Just adding extra vertices is an option of course, but as user I'd expect to get all loose edges as well, right?

diff --git a/io_mesh_ply/export_ply.py b/io_mesh_ply/export_ply.py
index 8de5d674..084c39b8 100644
--- a/io_mesh_ply/export_ply.py
+++ b/io_mesh_ply/export_ply.py
@@ -131,6 +131,13 @@ def save_mesh(filepath,
                 vert_count += 1
 
             pf.append(pf_vidx)
+    
+    # Loose vertices
+    ply_mesh_indices = set([v[0] for v in ply_verts])
+    if len(mesh_verts) > len(ply_mesh_indices):
+        for vert in mesh_verts:
+            if vert.index not in ply_mesh_indices:
+                ply_verts.append((vert.index, vert.normal[:], uvcoord, color))
 
     fw("ply\n")
     fw("format ascii 1.0\n")

By just appending the vertices to ply_verts we loose the actual vertex order. Problem is, without having that in place, I don't see any chance to support loose edges. Even if there is one single loose edge, then all edges (including that ones of the mesh) need to be written to the file as well as referencing to the corresponding vertices in the file, which is the main reason I'm using a dict, since we can get the order for free, also we can save some lines if the object has no UV's.

I also did some speed tests with this Ramose Murex Model (added one extra vert and one loose edge) and found out that the patch is even faster writing the file on my machine:

Current Exporter

Patch

Anyway, just let me know if you have any better idea or I can do something else.

Cheers,
Christian

Updated the patch for Blender 2.8x.