This additional constraint enables adding a route to the template (and p...

...otentially to the UseMvc method) without actually implementing the actual artifact.

For example without adding an area to a controller, a route can still be added to the template.
k @rynowak wins on this one.

Interestingly enough for the sake of solving the {areas} problem, we could have just written literally the simpler constraint that just makes sure a key exists.