Best practices replaces upward communication with events- when is two way binding appropriate?

[Darin Hensley]

I have been doing research for Polymer's best practices. Here is what I have so far:

1) Sibling elements do not need to know about each other. They should communicate between a master element:

                          element Master

                          |            |

                     element foo      element bar

2) for downward directional flow, using bindings. So, when element master communicates with element foo, use a one way data binding:

                      <element-master> 

                         <element-foo foo-property="[[masterProperty]]">

                         <element-bar bar-property="[[masterProperty]]">

                      </element-master>

If `element-foo` or `element-bar` send data back upwards, use emit a event. 

This model eliminates the use of two way binding because upward data flow is using events instead of 2 way binding.   My question for #2 is, when is it valid/proper use to use two way data binding?