When an element is removed disposed (Element.dispose) it is removed from the DOM, but remains an active element that can be used and injected into the DOM again. The destroy method takes it completely out of the memory and you can not inject it or reference it any more.
For reference http://mootorial.com/wiki/mootorial/04-element/00-element http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29
