Yes, it seems a bit backward for Android to ignore the ext4 format, considering the roots of their operating system, but what advantages would ext4 have over VFAT when it comes to externally mounted SD cards?
Because of the finite write limits of Flash memory, journaling is a bad idea and will never be supported. What about overhead? Is there an advantage in overhead for the ext4 system vs ext2 or VFAT?
There are legitimate reasons to have ext4 as a file system manager on a computer. These include having the ability to use larger, more complex file names, journaling, greater capacity storage media and hugely nested directory tree structures. None of these advantages can be reasonably translated onto a (relatively) small storage medium such as an SD card, which suffers, by its nature, a limited life span (finite writes).
Yes, you could use ext4 on an SD card, if you hack it right; you have proven that. However, there seems to be no rpactical reason why Android should support ext4 on SD cards.