The term "pure Android" has always been used incorrectly, as pure Android is nothing except the AOSP code, the same code that CyanogenMod and other modders use to make ROM's for devices. But even CyanogenMod isn't "pure Android" anymore, and hasn't been pretty much from day one, because they've also added new features to the original AOSP image.
The Nexus devices have always been considered "pure Android", even though they weren't, and no Android device could possibly be, as long as it includes Google's own apps and services, which are definitely not part of "Android". They are Google's apps, just as much as S-Note is Samsung's app on the Galaxy Note devices.
It seems that with the Nexus 5, Google is moving in the direction of having an entire layer of their own on top of "Android", or that is integrated with Android, probably not unlike how Google has Chromium, the open source project, and then Chrome, which is Google's own product, based on Chromium.
One way in which it has done that with KitKat on the Nexus 5, is how Google Now is integrated into Google's own separate launcher (that is the default on the Nexus 5), while another is how the hot word for Google Now is "Okay, Google" only with Google's launcher (at least for now).
The Nexus program is about pushing the next generation of user experiences across software and hardware. As part of that, we're trying something new on Nexus 5 and we'll see how users like it. Users have loved launchers from Google Play for some time now; in fact, some of the most popular launchers have been installed over 100 million times. For now the Google Now home screen integration and "Okay, Google" hot word is specific to the N5.
I think Google implemented Google Now in a very smart way in the left screen, because it can be easily accessed from there, and I could see people wanting to install that proprietary Google launcher on their HTC One's, Galaxy S4's or Xperia Z1's, especially people who don't like the launchers of those companies as much and were already using some other 3rd party launcher.
I think the Chromium/Chrome strategy is exactly the right one for Google to pursue in regards to Android, too, because that allows them to keep building the open source Android and its open source apps, like they did for the phone dialer, e-mail app, and the photo editor (despite what others may have said about Google abandoning the open source parts of Android), while at the same time they can do their very own Google thing on top of both Nexus devices and Motorola devices, and allow people to experience Android the way its maker intended it, and as a real alternative to all the other OEM-centric custom versions of Android out there.