It is so funny. I started this thread. I have read every post with interest. I am a Google Play Music (GPM) subscriber and now further understand why Roon cannot currently integrate that and many other streaming services. That being said, my biggest problem is that I like GPM, and many of my friends also have it and we share playlists, so I am not giving it up.
I have realized that this Chromecast Audio (CCA) integration, which at first I felt was an unecessary addition to Roon, might actually be the solution I was looking for.
My current setup is several RaspberryPi Roon zones, many are connected to a whole house amp (12 channels) connected to some in-ceiling and some bookshelf speakers. Also I have a couple dedicated spots, one of which has (to me) the expensive stuff. My problem is that when I have friends over, we fire up our GPM playlists. Of course, I canāt play them in Roon 
I subscribed to Tidal, and used a service to transfer the playlists over from GPMā¦ok, this is a workaround, albeit an ugly one since we are frequently adding to and changing those GPM playlists.
So in thinking a lot more about this whole mess, it came to me. Buy a bunch of Chromecast Audios (On sale for $25 right now) and replace my less important (sound quality wise) Raspberry Pi zones with CCAs. Now, when I want Roon through my house, which is usually the casual listening, my CCAs will be the endpoint zones. When I want to listen to my GPM playlists, then I can cast directly from GPM to the CCAs. When it is critical listening time, I will keep my couple Raspberry Pis in the non-house amp locations.
Finally, I can cancel Tidal and save that $20 a month. Yes, I will miss the high quality streaming, but the reality is that 99% of the critical listening I do is from my ripped CDs and hi-res purchases.
This plan is far from perfect, but it lets me leverage the CCA hardware at each (most) endpoints for both Roon and Google Play Musicā¦or any other streaming music service that is not on Roon but allows google style casting.
So I here I am, the thread starter, who was complaining about what I perceived as a wasted effort by Roon to add Chromecast devices, and now I am going all in 
Thanks to everyone for the great insight in this thread!
P.S. I will be selling some nicely configured Raspberry PIs with various DACs soon!