Same quantity of light (if you exclude transmission losses), resulting in a brighter projection on a smaller area. If you take advantage of the extra light by lowering the ISO for a higher S/N ratio, remember that the cropped image requires magnified further to match the size of the full frame, which lowers the S/N ratio. Back to square one.Such an adapter would not be practical for two reasons: first, because the image circle is reduced in proportion to the conversion scaling factor, and because most if not all EF lenses are designed to only project an image circle roughly large enough to cover the 24x36mm imaging sensor, the result of such a converter in an EF-to-EF system would be large sections of black, unexposed sensor in the image periphery.
Naturally, but note that many people don't mind cropping (D800/600, probably many sony models will do it in camera), and may be willing to give up peripheral pixels in exchange for brightness.
That said, there likely isn't a major market for it.
Instead, what you've ended up with is a smaller viewfinder, worse AF, less light transmission, more aberrations, less resolution, more post processing, and of course more weight, size and expense.