what problem does this solve?

It is in the todo list (runtime/doc/todo.txt).

Some fonts either have too much spacing or too little spacing between letters. This allows users to adjust how tight or loose character spacing is.

And where is p_columnspace used? I only see the basic initialization, but it is not used anywhere. What sense has this?

This is just to get the option into the editor so other GUI's can use it. Specifically, we would like this in mccvim: macvim-dev/macvim#321

That would have been useful information that should be mentioned at the description.

I think this should possible also be usable by at least the gtk and windows builds. I am not sure about a new option, can't we use the 'renderoptions' for that?

I'm wondering if the proposed way -- adding pixels between wrongly designed glyphs -- addresses the following concerns:

1. How to assign pixels For left-to-right sentences, which side are additional pixels given? Or, are they given evenly on both sides? What to do when the given number of pixels is odd? What if right-to-left sentences?
2. Font Substitution The proposed way could be OK for some Latin font faces having wrongly designed glyphs. But what happens to font substitution? Does the proposed way work well for double-width characters, symbols. or emoji?
3. Rendering Engine Dependency For some rendering engines, it looks possible to specify additional spacing by the number of pixels. But other engines prefer constants such as condensed, normal and expanded without or with prefix semi, extra or ultra to the number of pixels. Which is the better way to specify additional spacing? Can we make a unified interface or option for that for our various GUIs?

I think those concerns have kept an idea like "Add spacecolumn option" staying in the TODO list.

1. Either side should work.
2. I'm not sure how to handle this.
3. Fonts already render differently on various rendering engines. columnspace just allows the user to fix these errors. If the engine doesn't support pixel spacing, it can map a pixel value to its internal constants.

This is not useful without any GUI actually using it.

No update for a long time, therefore closing.