please see demo - notice that foo is not assigned yet there is no exception in console. Per $compile docs exception should be raised, since foo is non-optional:

If the parent scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior using =? or =?attr in order to flag the property as optional.

nice to have: IMO attributes should be treated in the same way: required unless ? is used