As discussed on IRC, the smooth palette plugin's actual usefulness is quite questionable because the import palette can already create a new palette from an image, and do it better:

- it has the option to "merge the sample" whereas smooth palette's current version only sample from the active drawable;
- it orders by most used colors first whereas smooth palette ordering is absolutely broken;
- smooth palette has this phony "depth" parameter, which is just a number of try to reorder and is completely unnecessary;
- the palette import actually checks every pixels and keep the most used in the paletter whereas smooth palette use a random function to only sample a few pixels, so every call returns a different palette;
- import palette actually returns a real palette, not an image.

I think the only interest of smooth palette is being able to create an image, which may be useful sometimes (if well done). Some artists like to have their palette colors directly on the canvas on some layer.

My opinion is that this option could be implemented as a new action "Palette to layer" which creates a layer in the current image, available in the Palettes menu. This way, it can be used on any palette (however they were created).
Then we could just get rid of the smooth palette plugin.

The second question is whether there is any interest to have a generic GEGL operation to extract palette colors (cf. bug 749220), which we would then use inside the Palette Import code.