My thoughts on this, is that, those that would need a GUI installer, would not even consider installing Windows, therefore the Windows partition would be "factory" installed. Being factory installed would most likely mean that the whole disk has been used. So the Windows user would be faced with shrinking their Windows partition.
On other Linux distributions, typically they are turnkey, therefore would already have a boot loader, and likely more robust, e.g., grub. I wouldn't want to be the tail trying to wag the dog.
Perhaps I was being too Pollyana with the effort not worth the result, At least in the initial version of the GUI installer. Afterall the extlinux boot loader only applies to primary partitions.