Are you speaking strictly about Nexus devices? Cause I think you know the answer if you're including carriers/OEM devices.
If you are strictly talking about devices, while other Nexus devices may have to wait a few weeks, at least they get all the features that come with it. A few weeks isn't a big deal as it rolls to all Nexus devices (+ GPE phones).
Also, one of the biggest difference is that Google updates their core apps and framework individually. This is a huge advantage as you get updates throughout the year. In the past month or two, they've updated a ton of their core apps, and more is to come in the advent of Kit Kat. And this applies for a wider range of Android devices, not just Nexus.
Also, did iOS 7 come to the iPads the same day? I thought it was a little later?
Anyway, here's the grid:
iPhone 4S, but does not support:
Filters in Camera app
AirDrop
iPhone 4, but does not support:
All items from iPhone 4S, plus
Panoramic photos
Siri
Third-Generation iPad, but does not support:
Filters in Camera app
Panoramic photos
AirDrop
iPad 2, but does not support:
All items from 3rd Gen. iPad, plus
Filters in Photos app
Square photos and videos
Siri
You win some and you lose some.
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Is that really the reason? If so, I can forgive a 2-3 week rollout time frame to keep their Nexus devices affordable. Easily forgive.