If you're buying an AV receiver these days, one of the main decisions you'll need to make is whether or not you care about built-in AirPlay. It's an easier call if you don't have any iOS devices, but if you have an iPhone, iPad, or iPod Touch, you'll definitely want to think about how you'll be using Apple's wireless music-streaming platform with your AV receiver.
It comes down to this: is built-in AirPlay a must-have feature on an AV receiver or should you just add AirPlay functionality through a $100 Apple TV? It's a tougher question than it seems on the surface, as each method has its advantages.
Advantages of built-in AirPlay
Doesn't take up an HDMI input
HDMI connectivity is about the only thing that matters these days, so every port counts on the back of your AV receiver. If you've got a basic home theater with just a few components, this isn't a big issue, but home theater enthusiasts can easily run out of ports quickly. Owning a DVR, Xbox 360, PS3, and Roku box already gets you up to four, which would max out the Yamaha RX-V473, which has built-in AirPlay. Of course, losing that extra HDMI input matters a whole lot less if you're going with the Onkyo TX-NR515 (no AirPlay), which has eight HDMI inputs, including one on the front panel.
This is easy to overlook, but you'll only be able to play back AirPlay audio in a second zone if you have an AV receiver with built-in AirPlay. Second-zone functionality on AV receivers typically doesn't support any audio received over digital inputs (including both HDMI and optical/coaxial), so you're out of luck if you want to pipe your Apple TV's audio to a second room.
Built-in AirPlay allows this flexibility, and in some cases you can even listen to a different audio source in the main zone while using AirPlay in the second zone, or vice versa. (Some 2011 AirPlay receivers didn't allow you to mix and match sources in the second zone. I'll be testing that in 2012.)
No extra box: Less clutter and simpler
Sure, the Apple TV box is about unobtrusive as it gets, but it's still another HDMI cable and power cable cluttering up your home theater space. Perhaps more importantly, if AirPlay is built in, your receiver will automatically switch to AirPlay mode once you select your receiver to stream music -- no need to switch inputs. It's no sweat for techies to switch inputs to use AirPlay, but built-in AirPlay makes it a little easier for someone who might be intimidated by an AV receiver remote filled with tiny buttons.
Advantages of adding AirPlay via an Apple TV
Full functionality of the Apple TV, including video
This is by far the biggest advantage of using a separate Apple TV box. Built-in AirPlay is an audio-only affair, while the Apple TV lets you stream TV shows and movies from the iTunes Store, play back your iTunes Match collection from the cloud, view photos from iOS devices, and stream Netflix, MLB.TV, and a few other sources. And with more features like mirroring coming to OS X Mountain Lion, the Apple TV is only going to get better.
Better user interface for cover art
AV receivers typically will display cover art, along with song and artist information, onscreen while using AirPlay, but it doesn't look nearly as good as the Apple TV's user interface. If you're planning on putting music on during a party and you want some eye candy to leave on the TV, you're better off with a separate Apple TV box.
More likely to get updates
If any new features are added to AirPlay, you're nearly guaranteed to get that functionality on an Apple TV. If you have a receiver with built-in AirPlay, it's much less likely that manufacturers will continue to support the product, especially after it's been out for a year.
So should I care about built-in AirPlay or not?
Unfortunately there isn't a simple, one-size-fits-all answer. It all depends on your specific setup and home theater preferences.
Personally, I wouldn't weight built-in AirPlay that heavily if I were buying a new AV receiver. But that's specific to my situation: I don't use second-zone functionality, I occasionally buy movies and TV shows via iTunes, and my Logitech Harmony 650 will take care of changing inputs for me. So if I were choosing between the Marantz NR1403 ($400, no AirPlay) and Marantz NR1603 ($650, built-in AirPlay), I'd get the NR1403, buy an Apple TV, and save $150. (That's an oversimplification since there are other differences between the NR1403 and NR1603, but the general point remains.)
However, I can imagine a lot of buyers with different priorities for whom built-in AirPlay would work better: if you use the second-zone playback, don't care about the Apple TV's other features, or just don't want to deal with another box.
The good news is if you want built-in AirPlay, you don't have to pay as much for it this year. Built-in AirPlay is available on several more affordable receivers this year, including the Denon AVR-1613 ($400), Yamaha RX-V473 ($400), and Pioneer VSX-822-K ($380).