You’re probably thinking why I have model with only edges and no faces, I need that kind of data to export to another tool.
Anyway, I can’t get “with automatic weights” option to work for my wire (wireframe) models. But “with envelope weights” option works. Am I doing something wrong, or is this a bug or limitation of “automatic weights” mode?
I was wondering so I just tested.
Apparently, yes… Parenting with automatic weights doesn’t work without faces. I don’t know what fails exactly. It seems that the vertex groups are created but I didn’t check further to see if they were correct.
I guess that weight painting won’t work either.
But that’s not a real problem for what you need. Just keep the faces until you export. Well, at least until the weight painting is done. The Armature modifier itself doesn’t need faces. It works just fine.
It kinda is a problem. I wasn’t specific: I create line abstracts used by another program. They aren’t made up of polygons inside Blender as well.
Yep! That’s a problem… and I don’t see any solution. I tried to add a Skin modifier to create “artificial” faces (The first crazy idea that went thru my head.) but it doesn’t work. Without faces, the Vertex groups are just empty.
Flash!
Second crazy idea: Select everything and extrude without moving. [A], [E], [RMB].
Now, it works!
(Little problem that I encountered is that the mesh becomes invisible in Object mode if it isn’t selected. Don’t know why…)
And when you don’t need the faces, [W] --> Remove Doubles.
Anyway, I’d report it as a bug. Maybe it’s fixable… Maybe not. It depends on if “Parent with Automatic Weights” really needs the faces for some reason… or not. And vice-versa. 
The extrusion seems like a fine temp solution. Nice idea.
I think this is likely a bug myself. Is there any bugtracker for Blender so I can post this.
Another problem I found which isn’t related to this topic but related to edges not forming a polygon: they can’t be vertex painted or display colors…
I see what you mean.
It might be impossible with the algorithm used by auto weight mode.
But it can also just be a bug nobody noticed till now because of little use case.
So please link to this topic.
Your solution could work if I just wanted to export a wireframe of a polygonal mesh. The thing is my models are “only edges” from the beginning:
That’s what I thought. But then, workarounds are all clunky.
Out of curiosity, I wonder if you could rig your stuff with a meshdeform modifier instead of trying to manipulate the bare edges directly with bones. I have no idea how well it would work, but it might be worth a try.