Originally Posted by LWaB

The adaptor pin is designed to reduce the folded size by positioning the saddle further forward. Putting the saddle further back will increase the bending moment applied to the seatpost.

The telescoping seatpost uses a thicker wall and I believe the extended seatpost does also, both of which will minimise flex. There is supposed to be a factory option (rarely mentioned) for stronger seatposts for those who bend Brompton posts.