Considering the following code:

var appendTo = this.$.myCustomElement,
  t = document.createElement('template', 'dom-bind'),
  span = document.createElement('span');
span.innerHTML = '{{hello}}';
t.content.appendChild(span);
t.hello = 'hey';

Appending this to the light DOM won't work:

Polymer.dom(appendTo).appendChild(t);

This will only render the template tag with the content within and data-binding placeholders emptied.
The dom-change event will actually fire and informs me that the attached() callback stuff of the dom-bind element actually have been run.
But stamping fails and moving the content outside the template never happens.
Tries with dom-template and manual stamp() also failed.

It actually needs to be in the local DOM:

Polymer.dom(appendTo.root).appendChild(t);

Is this a bug or by design?
Could not find any documentation on this.