It's been added because Apple has made it so dragging to the top of the screen now activates mission control, and as this was the normal method for opening the Dropzone grid this caused problems. Also people often forgot they could drag to the top of the screen, so seeing Dropzones window up there reminds you 'hey you can drag this item here'
But you can disable this feature by opening a Terminal and copy pasting the following command:
defaults write ~/Library/Preferences/com.aptonic.Dropzone3.plist ScreenTopDragEnabled -bool false
And to re-enable:
defaults write ~/Library/Preferences/com.aptonic.Dropzone3.plist ScreenTopDragEnabled -bool true
The command is slightly different for the Mac App Store version of Dropzone. For that version the command to disable it is:
defaults write ~/Library/Containers/com.aptonic.Dropzone3/Data/Library/Preferences/com.aptonic.Dropzone3.plist ScreenTopDragEnabled -bool false
If you disable it then you can still drag onto the Dropzone menu item.
A shortcut you can hold to show the overlay is a cool idea. I will consider adding this in a future version.