Why is it not possible to grab a file with the mouse in filebrowser, then, at holding it, press alt-tab to paste it in another window?

In gnome flashback/2x/metacity this worked nicely, and I really miss that. I always have to resize one window so I can drag the file over the other, or drag the file onto a starter icon...

Also because I use window grouping at the panel window buttons, I am not able to drag the file on the window buttons which are grouped, as this also does not work. Both of these hinders workflow a lot.