I'm trying to figure out the advantage of Plex on an ATV, vs what I do now, which is running Plex on a dedicated Mac Mini hooked to my TV. The only thing I can think of is that its one less activity button on my Harmony Remote.
For me there are a couple reasons I dropped the dedicated HTPC and picked up an Apple TV.
1) It is way easier to use for family and guest. Granted I wasn't using anything like Kodi or Plex back then, it was still just not as clean of an experience and most people gave up using it.
2) Smaller footprint in two areas...space on the media cabinet and my electric bill. I was using a old comp for my device before and keeping that puppy anyways one was draining, literally.
3) Noise! Hearing that fan spin up in the middle of a movie was a huge annoyance. The Apple TV is silent.
Do I miss tinkering with a full blown PC on my TV? Sure, but I found that thinking with the Apple TV is just as much fun, and the new App Store will add to it.