If the kernel is that of HTC/Samsung as opposed to that of Google (i.e. Nexus devices), who is to say that the updates will truly come straight from Google?
In order for a phone to be updated in the Nexus fashion, CDMA GNexi aside, it must come from Google, and is to have never been looked at by an OEM or carrier. Sounds like the OEMs will be involved to a certain degree.
Am I saying that these devices won't get Android 4.3/5.0 in a timely fashion? No. But I wouldn't count on these "Experience" devices to be treated as Nexi. Just look at the Xoom.
Nevertheless, we have been waiting for flagship devices to ship with the Nexus treatment, and this is about as close as it gets.
jmar