Everything else I use from multiple devices.
Using Facebook (or any kind of login) to solve that would be a bad idea (but many might be tempted to do it).
However, in those cases when the task is more complex than that (I just checked, that applies to eleven† of all 23 third party apps I have installed on my iPhone) that’s not a solution. I – the user – want to be able to access the data from anywhere and do actually use that functionality regularly.
Honestly, I personally would have no issue if each of those apps gave me a choice of signing up with either Google or Facebook. That would massively simplify things for me and I wouldn’t have to lug so many accounts around. I can understand the privacy concerns, so I do see the need for alternatives, but for me personally it’s not an issue.
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† Reeder, Twitter, FB Messenger, iBooks, Pages, Keynote, Numbers, YouTube, Twitch, Vine and Dropbox; Apps that already don’t force me to log in, most because it’s just not necessary: Castro, arte, Star Guide, six game apps; Apps that force me to log in for some functionality and I’m not sure how I feel about them: iPhoto, iMovie, Remote.
‘Often’: many websites don’t see this; I’m willing to bet those I use regularly (StackOverflow, HN) aren’t even close. Most analyst describe a currently slight majority for phone metrics (except time on the site); more importantly, demographic elements suggest phone attention share will grow.
> I'm truly sick of hearing that the phone is the only device that matters.
‘Only’: OC hasn’t said exactly that -- he talked about authentification; most people don’t really say that either. Many journalists exagerate a trend and slight majority, but I would (tongue-in-cheek) blame you first for reading badly written magazines.
My experience is that authentification is safer for non-developer users on their smartphone: you have a lot more stream for consistent two-factor authentification; physical security is better for something in your pocket most of the time; it’s the only random-key generator (capable) most people have close to them. Roughly half of users users actually lock the damn thing…
Most people are going to access your app from one device, their phone. The phone is the one true source of identity. Let us all be glad that a privacy-paranoid company such as Apple sets the standards there, rather than what Facebook would have liked, had their phone efforts worked out.
I will also note that taking advantage of device token for "login-less anonymous use" is way way smarter for developers, from a usage perspective. Every single tap or interaction is a point of friction for your users; just eliminate them altogether. This is, for example, why TouchID-based devices generate more App Store transactions than devices based on the normal password system.