Having to stick to only Nexus phones is a gigantic blow to the platform's appeal because it removes the hardware choice that is one of Android's main selling points vs iOS.Hurt developers and you hurt users. When developers are burdened with avoidable bullshit like the glacial adoption speed of new OS versions, the users get less software, and the software they do get is worse.
If always having the latest and greatest version of Android available is a big deal, go with the Nexus line. If it's not go with whatever you want or jailbreak and run custom ROMs when they come out. It's like complaining about iPhones locking you into a walled garden when it's not terribly difficult to get out of it if you want.