The example is extras1&2 in extras area.
When extras has configured inheritance=true, the sub areas extras1&2 should also inherit automatically.
Only if a sub area explicit configures inheritance=false, it should not also inherit.

This should be this way cause of:
Otherwise you loose control by the prototype. Any concrete template can have more than 2 extras areas. And theoretically any concrete template can have in any area as many subareas it needs.
When in the prototype an area is defined as being inheritance=true, this should control every of its sub areas within this area which is not explicit overruling.
And additionally, id a sub area always need also to define it, you can't have the concrete template react dynamic to the different prototypes.