Thats what I do. It works great. The Android TV built-in chromecast leaves A LOT to be desired.
For Example:
Netflix casting
(Apparently recently fixed after a few years)
weather casting
calendar casting
pretty much any information casting
With a real chromecast you can set hz to 50 instead of 60. That is a must for A LOT of people (25/50 FPS countries), especially if you live in EU and watch sports.
Just announced by Nvidia for shield TV https://shield.nvidia.com/blog/shield-google-home-October2018?ncid=so-twi-slglhmmn-61000
Why isn't Android TV running exactly the same software as a standalone Chromecast device? That makes no sense.
Seems highly unnecessary. Which features would that be?