A while back I joked on Twitter that if Apple put filters and tilt shift into the built-in Photos app, I'd never use Instagram again. A lot of people maintain Instagram isn't about the filters or effects, but the community, but frankly there are already way too many communities for me to keep track of, and I'd just as soon make as many as possible as redundant as possible.
You can say adding filters to Photos.app is skating to where the puck was back when the NHL was still skating, but any time basic functionality is baked into the OS, at a basic level, it reduces complexity. In iOS especially, where you can't alter the default apps, and the built-in iOS camera is available at the swipe of a lock screen icon, and the Camera Roll can post to Twitter directly, and Facebook, and more, that's where I want basic filters and effects to live.
It's Camera Zero. Everything else is an alternative, an extra step, and the more they're a value add and not a must-have, the better.
Look at only some of the current slate of filter (or filter-and-share) apps. From top left: iPhoto, Instagram, Twitter, Facebook Camera, and Flickr. iPhoto's interface is overly involved and complex for people who just want to futz with and share photos, and none of the third party filter-and-share apps do simple interface as well as Apple. And all of them require more steps then if the functionality was included in the core OS.
Apple had video effects, for good or for ill, in the 2009 iPod nano, and Apple included filters in Photo Booth for the iPad, so the core concept isn't without precedent. Android includes filters in the stock Camera app as well.
Basic editing was added to the Photos app with iOS 5, shifting filters and effects there in iOS 7 there makes the kind of sense that does. In fact, Apple could build in live filters and effects, since they have full access to every atom and bit of the machine, something not afforded third party apps.
Once a filter and/or effect is applied, Share sheets would make it a snap to send to not only Twitter and Facebook, but iMessage and Photo Stream, or to set as a wallpaper. It becomes OS-centric rather than app-centric, and anything that gets added to the Share Sheet would get to source photos with filters and effects "for free", in one step.
Online photo ownership is a huge battleground, with entrenched players like Facebook/Instagram, Google+/Picasa, Yahoo/Flickr, and Twitter all trying to make their platforms stickier, and hedge against social migration, by making the things we care about, especially photos, fun and enjoyable on their networks. But they're often dicks about it.
With basic filers and effects built into Photos.app, if Instagram blocks Twitter, if Twitter blocks 3rd-party clients, or if Facebook or Google tries to assert copyright on my photos, I don't have to worry that I'll lose my ability to make some of the best mobile photos on the planet look like polaroids from the 1960s. I'll have iOS 7. I'll be fine.