Why should it be included?
I own a wi-fi adapter with a RealTek chipset that requires this kernel module to function properly, and compilation on every kernel update is tedious. I'm sure there are other people with similar adapters who could benefit from this being included in the repo (here's a list of all the adapters that need this module from wikidevi.com)
Is it open source?
Yes, at https://github.com/gnab/rtl8812au.