Otherwise we will be in situations like a friend was cursing me yesterday about how I dare have android 4.2 (and wait for 4.3 rom) on my unlocked 2010 device and he is stuck on his HTC with 4.1 and no signs of incoming upgrade soon.
I understand the necessity of the "rolling update" model in general, however in the case of Nexus machines it should not be necessary. At any rate, I should be able to opt out of the rolling update model and put myself at the front of the line without having to manually flash a factory image.
Caution though, apparently this breaks app updates for some people, but all you have to do is add and remove your google account to get them to work again.
For example, if my application depends on some feature delivered in 4.2, I need to be aware that my potential market is less than 5% of devices in the wild.
On the other hand, if the app is 100% compatible with the 2.3.X line, I can count on the all of the devices working, but I also should regression test with the updated versions to make sure that they work.