There is an extremely useful image editing technique currently not possible in
The Gimp. The Rubber Stamp tool in photoshop (since at least 3.05) has a "sample
merged" option. This will allow it to take its cloning samples from all visible
layers of the image, but only write to the current layer. 

It makes image retouching *VERY MUCH* easier and more fun. 

Currently, The Gimp will only clone from a single layer. This becomes a problem
if one has multiple edits across several layers.. it's impossible to clone from
the image without merging them. It also makes it extremely difficult to use
cloning effectively to edit an image on a "clean" layer, while not touching the
original image. You cannot clone from both the background layer and the
"currently editing" layer at the same time, a common technique for blending.

The "sample merged" option should only clone from the currently visible layers
in the image. This gives control of what you clone by hiding / unhiding layers.
It would appear as a checkbox, "sample merged", and can be turned on or off as
desired. Thank you.