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I prefer to put everything on my iPhone, and proven that all my imported music (except one) are lossless format, 128GB is barely enough to store all of them. I now can store all of them because library is not so large. But soon, it will exceed 128GB and I will need to choose the one I want to listen when syncing device.
I think iPod would be very capable to be used as a test device. Yes.
I hope Apple can release an iPhone with 256GB or even 512GB storage so I can once again carry my whole music library with me.
 
Consolidation of devices.
I used to carry a PDA, a phone, and a CD player.
Then the CD player becomes a smaller MP3 Player.
Then the phone is good enough to be also be an MP3 Player.
Then the PDA functionalities are built into the phone.
Then I just carry my iPhone.

Another advantage, I don't miss my notifications while listening to my music since the phone is also the music player.
 
Easy, I want to carry around as few devices as possible. My iPhone does a good enough job of being an MP3 player, and it does more. Don't need to duplicate work.

I'm kind of the opposite. My iPod touch has my full library on the device but my iPhone and iPad just pull songs from the cloud using match as needed. I like having one dedicated device that can hold an entire library. To each their own I suppose.
 
At some point, the only reason I could eventually think of for using a music-only device was its light weight (like, using the almost-no-matter third-gen iPod shuffle on walks or jogs). But now the Watch can even do that, so gaah, life is too incredible these days for this even to be a deep question anymore.
 
I like having both an iPhone and iPod touch. I load all my music and videos and games on the iPod touch and use it while working out and at work.
 
I wish apple invest a little bit and not just use last gen production block so we can have extra buttons to change songs like how Sony did it to their players or sandisk.

Hate to wake up the device everytime I want to change songs. I maybe outdated but I checked the apple store and gen 5 / gen 6 are identical.

Why not use the earpods controls?
 
I don't really want music on my phone - I can stream if needed.

I like the iPod Touch 6 for Google Play Music and Health.
 
I currently own a Samsung Galaxy S5 and I don't like how the music is organized on there, so I bought the 6th Gen iTouch and put all my music there. Plus, I'm able to use the controls on my PowerBeats Wireless 2 on the iTouch and not on the Samsung since I'm always required to skip the song through the phone which can get very annoying when I'm working. I like how Apple organizes my music better.
 
More notably, dedicated MP3 players like the Ipod Classic, or various ones by Sandisk, etc.? Is it because you also want a device that can access iOS?
Maybe it has to do with whether you were in the habit of carrying your music around before the iPhone came along? I wasn't. When walking around cities and suburbs, I prefer to be in contact with my environment, rather than block it out. Fundamental safety. In the forest, I'd rather hear birds than Resphghi's The Birds; Schubert's Trout and Beethoven's 6th are more evocative when I'm not surrounded by nature.

If you're the sort who prefers to leave the phone behind; if music is your refuge and escape, then clearly, a separate music player makes sense.

I have a hard time understanding why anyone would carry two items in their pocket when they can carry just one, unless they generally carry either one or the other, rather than both.

If you're the sort who wants to have your information, communication, and computation available at all times (and that information and communication device also includes a dandy music player), there seems little or no point to carrying a separate music player.

I love the integration. If I am listening to music and my phone rings, or Siri is providing turn-by-turn driving instructions, the music pauses and resumes automatically. The same headset can be used for listening to music and answering a call. Since I have a nearly constant data connection, if I have an uncontrollable urge to hear a piece of music that hasn't been pre-loaded... I've got it.

I'm not sure what "access to iOS" means to the OP. iOS is an operating system; a user interface. Tabula rasa. It's a means to an end: Access to telephone, camera, text messages, email, web sites, news, stock prices and sports scores, banking, reading matter, journal, calendar, games, photo album, pocket calculator, guitar tuner and songbooks, business documents, audio recorder, dictation device.... This is an incomplete list of the features I routinely use. And in most cases, the built-in music player will continue along on its own, without interruption. When it is interrupted, the interruption makes logical sense. So yeah, if that's "access to iOS," why would someone not want it?
 
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