Why do you think they’re meaningless? They show the corner of a mesh, first perfectly sealed, then with the top blown wide open.
Maybe a little illustration shows it better:
I’m running Blender x64 r52813 (nightly build), that should be shortly before the 2.65 public release, on Windows 8. I cannot provide the .blend file because it is a commercial model, but it has been created in Blender (not imported), fits in a bounding box of about 50 units and contains >600,000 vertices.
I could try to create a reproduction case, but in any case, I think I might have already figured out what was going on: I applied the “Edge Split” modifier before the “Decimate” modifier, so that vertex was actually split into 3 (one with the for each face connected to it with a sharp edge).
It still surprises me very much that “Decimate” moves vertices around (versus only removing and inserting vertices), but knowing that now, I can arrange the modifiers accordingly.