I'm excited about having 4K video on my iPhone, but it will admittedly not be the default setting for me when shooting videos. Every phone I've had with 4K video has had some restrictions. One is obviously that you are limited to 30 fps, where with 1080p you can do 60 fps (or higher for slow mo). On my Galaxy S6, you cannot use stabilization or HDR when shooting 4K. I'm not sure it's clear yet which features will be available during 4K shooting on the new iPhone. We know it's 30 fps, and it appears that OIS still works (at least that's the way it looked in some of the hands-on videos I saw). And then there's the file size. I opted for the 64gb model, but even that will not hold much 4K when you consider the OS, apps, etc.
On the other hand, I will definitely be using it for certain situations. Here are some things to consider:
1) I love how you're able to zoom in during video playback to show a lot of detail. Even though the iPhone won't display 4K resolution, this is still an advantage to shooing in 4K. Is that alone enough to warrant the extra file size, etc? Probably not, but that zooming feature looked pretty nifty in the keynote.
2) In addition to zooming during playback, you also have a lot of leeway when editing 4K footage. You can zoom, crop, manipulate your footage to focus on certain areas of the video or create special effects. I have very little experience with this, but there are tons of articles and videos out there showing the advantages of this. I don't know if Apple's iMovie will be able to take advantage of any of these video editing features, or if they'll be added now that the iPhone can shoot 4k?
3) Also, I know at least on the Panasonic GH4, results of downsizing 4k to 1080p are better than native 1080p. I don't know if this will translate to the iPhone's video with its smaller sensor and likely lower overall quality. We'll have to see what bitrate Apple is using for both, I guess. But it'll be interesting to see some comparisons between native 1080p and downsized 4k.
4) There's also the advantage of future-proofing. It's not that 1080p will be going away, or even that 1080p isn't enough for most display sizes that people use today. But aside from the storage issue (and perhaps some feature limitations that we aren't aware of), shooting in 4k will mean having the best quality available. I personally don't mind dumping my photos and videos to the computer periodically, and I have a plethora of HDD storage of my computer, so why not shoot in 4k?
5) 4k TV's are coming down in price, and this might be the year when it starts to become mainstream. Last year during the holiday rush and Black Friday deals, there were a good number of 4k TV's being sold at decent prices. This year, I expect there to be a VERY strong push for 4k. And yes, one can make the argument that the difference between 1080p and 4k is difficult to distinguish with the naked eye at certain sizes and viewing distances even on a 4k TV, but there are more things to consider than just resolution. Again, I don't know how the iPhone's 4k will hold up, but 4k video footage holds the promise of not only higher resolution, but higher bitrate, better dynamic range, color, etc. Probably not with this year's iPhone, but let's see.