There might exist some other fingerprints that can lead to the knowledge of what browser you're using (the existence or non-existence of some feature, etc). If your User-Agent contradicts those others fingerprints, you become very easy to identify. It's really difficult to consistently pretend having a given configuration unless you are, well, actually running this configuration.
More generally, the more you use technologies to protect your privacy, the more you increase your fingerprint entropy, that is, the more it's easy to identify you.[1]
I wouldn't advice those "spoofing" tools, but rather to use the browsers' settings at the maximum of their capabilities (disable third-part cookies, delete cookies when closing the session, set Flash to 'Ask to activate', etc) and just the minimum extensions set to block third-part requests in a first-place (Disconnect, etc).
I don't see how pages transition is trying to emulate native apps. It still falls under the paradigm of accessing a resource at a given URI, which is what the web is. Personalizing the way the resource is loaded by the client and displayed to the user? That's just cosmetics we should definitely care about.
Currently, the most popular way of implementing "native apps" in the browser are Single Page Applications, which breaks the web paradigm and hence have practical drawbacks such as the inability to open some resources in a new tab or to bookmark them. One motivation of SPA is the fact that loading a new webpage produces a blank intermediate screen during downloading, which is lame, especially for pages which are almost the same. Pages transition can change that, and in a beautiful way.
Imgur is already bloated. As for reddit, I'm curious about what makes you think they're drifting towards bloating. They use JavaScript (replying to a comment creates a new textarea), AJAX (posting a comment), WebSockets (real-time updates of comment timestamps), modals (sign up), but they do this in a very moderate way and the result is really robust. It seems to me that they perfectly know the power of all these technologies but have a very strong QA which doesn't let a single shit get pushed in prod.
The thing with Pocket is that it's basically an extension installed by default. This is the first time Mozilla does such a thing to Firefox, and this doesn't please the community, including myself.
I though that the whole point of integrating a new feature is to really implement it into the software, where you can make it efficient and reliable, where you don't rely on a third party to make it work and where users' data is bundled to their Firefox profile only.
If they think it was not worth it to implement it, then they could just label the extension "featured", make a blog post to advice it to the community and call it a day. Otherwise they can just start integrating any popular extension, call it a new "feature" and defend themselves by saying that people use it so it's important.
Aren't most website storing long-term cookies when the user check the "remember me" function? Also, I think that the most difficult part of an automatic signup system would be captchas.