The rectangular hole does two things...

- It aids in wicking in the CA
- It lets the control surface move more easily

It does the latter, because there will be slightly less material full of CA to "bend" when the hinge moves.

The drawback to this is that the hinge is slightly weaker, but that difference may not be very important given the overal strength of the material used.... it is far more likely that you'll rip the wing out before you put enough force on the CA hinge to break it.