"Your Apple TV can't access your content on your NAS."
Ehhh... not true at all. If you bothered to read even portions of this thread you would've found that many of us use our NAS as our Plex Media Server, and stream it to our Apple TV's, without using AirPlay.
AirPlay is pretty wonderful, but it actually gums up the works if all you're trying to do is stream local content from one device to another. By using your iOS device as a middleman, you're effectively doubling the amount of data flying over your WiFi (once flowing into your iPad/iPod/iPhone, and then the exact same content flowing back out to your ATV).
Plus, my ATV is hard-wired to my home network, as is my NAS. Why would I want to use, let alone tax, my WiFi if I have a much more stable wired alternative. My WiFi is left free, my ethernet streams my media like a champ, and I don't have to sit and watch the batteries drain out of my iPad like I would if I was AirPlaying.
In addition... with my content streaming directly to my ATV, that leaves my iPad free for my kids to take to the other room and stream their own content (also from Plex on my NAS) simultaneously. No sense tying up the device if you don't need to.
DLNA is nice, too, but I've found the interfaces to DLNA clients to be incredibly lacking... most to the point of being completely unusable (to me). The interface of the PlexConnect client for the ATV is incredibly easy and very Apple-like. AND, as a bonus (if you can call it that), Plex is DLNA-compatible... so, if I ever got a hair up my craw, I could use the DLNA client on my SmartTV to pull content from it... but I never will.