Dragging messages from the shapes panel is really a slow way of drawing diagrams.
It would be nice to have a keyboard shortcut for dropping a message exactly to the position of cursor. (Thanks to message autosnapping, it usually gets connected to required instances.)
Define two keyboard shortcuts, one for each direction (left-to-right, right-to-left). The user should be able to invoke the shortcut just by their left hand (because they hold mouse in right hand, mostly, which specifies the location to drop the message).
Some similar functionality has been added in 0.5.4 release. User can use shortcut "Ctrl + Alt + x" to put down the last dropped object to cursor location.