_WeldVertices was not working.
Sometimes when you try to weld vertices of a mesh and they do not weld,
as shown in this video (subd extracted mesh)
using Rhin6 (Not Rhino7 WIP)
In Rhino 7 WIP the OBJ export is broken.
In this video, I convert a SubD to mesh or extractRendermesh (is the same).
I make two comply. The one in the middle I export it as OBJ and reimport it.
Then I apply _WeldVertices to both meshes. (I will lose UV mapping but is fine in this case)
As shown at the end of the video, the exported one was able to weld vertices
By default, WeldVertices is not welding.
So you need to export as OBJ and reimport it to make it work.
Mesh welding is necessary to reduce draw calls or for having a unique UVmapping with no island so that is possible to apply UV mapping outside. Mesh welding in Rhino is important because it preserves vertex normals.