I take it that the UV Editor is for mapping models and an Image Editor could be like Gimp but with Blender look and feel, with tools such as selections, lasso, crop, paint and clone brushes, pen, text… some of these already work in Blender but have a 3D look alike and operability in them.
Grease Pencil, Vertex Paint and Texture Paint are all functionalities apart from an Image Editor, from a Gimp like window. You can’t really create and edit a new image in Blender with all the settings and tools Gimp has to offer. Yet. I agree that Grease Pencil, Vertex Paint and Texture Paint do have settings and functionalities that do resemble or can be partially used for an Image Editor. That’s why it needs to be studied carefully. Felix_Kütt is right, keeping it simple is right, but hard.
It is something to consider for the future, and there are definitely more important stuff going on