My use case is a bit more complicated. I have a sound system with 7 Sonos units and 3 Video units - too much for an Apple system to manage without a lot more extra hardware. Apple TV also doesn't solve my problem.
Plus I also use the NAS for active file storage (mapped to the MBAs) and for passive file storage. In total some 9TB of working drive space over 5 drives (RAID5)
To be fair, the system did evolve over 5 years or so, starting with the active file storage. However, if I was doing it again I would still go the same way.
I guess that's fair enough, so you're using it as an active file server as well as a media server. I just use Dropbox and iCloud instead.
Buddy has a Sonos based system as well, 12 Sonos speakers throughout his property, outside as well as in, but he uses his venerable old tower desktop as a media server, with several external HD's in the mix, same for all his 8-9 smart TV's that access his home network directly (Samsung series 9 TV's I think? The guy is nuts about entertaining guest and having parties, like a 42 year old frat boy

- he's also got multicolor LED's wired up indoors and out, and he runs all his audio and lighting from his iPhone, as well as several IP security cameras, etc.) Doesn't have a NAS in the mix. His setup is obviously organic, and grown over time, much like yours.
My setup is more family based - catering to the wife and kids rather than having video and audio in every room of the house as well as out in the yard, so ATV's are sufficient for me. I also had the opportunity to assess my needs and do a one time setup for the whole house just the once instead of slowly growing the system over time, and conforming that growth to what I had done before. But again, my setup is extremely simple, I just want to be able to play anything in my video, audio, or photo library from one room on each level of the house and on iPads that we each have.
Like I said, I see the point if the NAS relieves you of having a desktop computer in the first place, absolutely.