For example, in their Getting Started page [0], they have an example of binding component data to elements. It appears that the Age slider is bound to the {{age}} value because its ID is "ageInput". I would prefer to declare the binding with a data-bind="age" attribute on the input, or something else equally explicit.
Does this mean that it is the value="{{age}}" attribute that is causing the binding?
What happens if I want to do something like: <input value="{{firstName}} {{lastName}}"></input>?
In practice, this almost seems like "sugar" that automatically binds an input to a value if it's value contains ONLY the property.
To test out the binding sugar, I tried using this input: <input value=" {{name}}"></input> (note the space before {{name}}), which did not bind {{name}} to the input.
I'm still of the opinion that the binding should be more explicit.
Edit: typo fix
> <input value="{{firstName}} {{lastName}}">
This will work as you expect (http://jsbin.com/ecejiy/6/edit). Plus, if either of those property values change, the input.value will be updated accordingly.
By the way, you can also add/define your own syntax for MDV bindings: https://github.com/Polymer/mdv/blob/master/docs/syntax.md