I will double ask for this one. This is such a key feature that would be great.
Sometimes you need to fade an arbitrary edge - not just top, bottom, left, right, etc
Else, I make a PNG that fades from transparent to black in Photoshop. Then I bring that into Quartz Composer and send it to MM via Syphon. It looks like black but the alpha is preserved and you can then mesh warp this and create a multi faceted fade.
Would be very nice in the program though!