Whenever any node is set as a leaf, you can't append nodes to it via drag+drop.

I think this must be a mistake since there are config options 'allowChildren' and 'allowDrop' to control such behavior.

Changing this line of Ext.tree.TreeDropZone in getDropPoint() (removing the isLeaf check) fixes it:
var noAppend = tn.allowChildren === false || tn.isLeaf();