There are times when you don't want to add all of the resource to something, such as when putting firewood in a campfire which you are going to leave behind or dismantle after a while. It could be a useful option though and has likely been requested many times over already.

It certainly would be nice if there were two commands, one to add all and one to add one at a time.

There are so many times you don't want to fill all. Better option which has been suggested is a fill all button, instead of only press e to add it should also have a option saying press (whatever keybind) to fill all