The reason why it is not possible to have a composite structure without any core fill is the following:
Walls using composite fills for structural representation can be slanted or even double slanted. Now, when changing the "Slant Angle"
it is the core which will define how the rest of the structure is going to look like.
The graphic below shows twice the same wall, only the core skin is different.
When changing angles, it is the core which is changing, every other skin is adjusting to it.
A profile wall can not be slanted ... its shape has to be defined manually, there is not such automized adjustment methods as there is for a composite wall. Therefore it can exist without a core.
By the way:
All complex profiles are set to be core components by default, the user has to switch of this characteristic consciously, manually.