This could be used to save on the use of gameface for most basic use cases or to render the same widget again and again in the world without instancing multiple widgets or requiring an additional render target.
Those rectangles should be able to be rendered both lit and unlit, with or without texture filtering and with and without translucency or with alpha test (alpha test can be used to render very detailed distance field shapes as seen on text in team fortress).
Could be able to use alpha channel as glow mask to boost by a numeric variable the actual rgb value.
Engine can really optimize the render of multiple copies of the same rectangle way more than actually possible using widgets to add images to the world.
an added plus could be the possibility to modify statically(at edit time) or dynamically (in game realtime) the uv mapping of those rectangles so that they can represent not only the {0,0} to {1,1} range but also being used tiled , mirrored or as a texture atlas by showing subrectangles of a bigger texture/widget.
p.s. On these rectangles there is no excuse to not support scaling 