What 3:2 pulldown? We're not doing interlaced conversion of film to TV here!
It's not too many options; it's just Apple being ignorant and refusing to admit it. Apple operates in a fantasy world, where they refuse to learn from anyone else and unfortunately they are often given a free pass on it. Canon made the same stupid mistake 10 years ago and got harangued for it until they fixed it. There's no excuse for Apple doing the same today.
Why would we want it? Well, not long ago I was shooting an interview at 24 FPS on my production camera, but then something started happening in the background that I wanted to get some B-roll of. So I whipped out my phone to capture it... but it was useless because it was 30 FPS and wouldn't intercut with, well, anything.
We are though. 24p is a film frame rate and often not supported by many set top boxes so call it a function of telecine, 2:3, 3:2 pull down, etc, whatever, except on a device that natively supports 24p output you'll need to take the 24fps and make it 60fps so it can be divided even to 30fps like this...
This can result in a stutter (technical term is 24hz judder) from the 7th frame, or lack thereof. Newer TV's are pretty good at compensating for 24fps from a 60hz source (
list). If you are producing video to be played on something that natively outputs 24hz like DVD and Blu-ray players it’s fine. However it’s a problem for many devices like set top boxes. This included the AppleTV which up until very recently had the same problem but got a frame rate matching feature with tvOS 11.2. But even then some people leave that feature off because it requires a source refresh (tv goes black like an input change for a second or two). Many people with Plex clients that don't support 24p often bring this up.
Not sure what you mean by 30 fps iphone video being useless. Most basic feature of any video editing software is accepting clips of various frame rates. iMovie on a 4 year old iPhone can do that right now. Actual video editing software will allow you to export at a preferred frame rate. Regardless you’d need some type video editing software to stitch the clips together anyway and whatever that is I'm sure its capable of various FPS. You can drop a 24hz clip next to a 240hz clip in the free iMovie on Mac and probably Windows Movie Maker (not sure if that is still around). Its preferred to do all this in post anyway generally at 30/60fps.
Although my main point is people aren't shooting b-roll with iPhones, dont have production cameras, and if so they have video editing software anyway making the topic a moot point.