Depends what you're using it for.
While your argument that two separate devices is a faff when they both live in a pocket and music is listened to on headphones makes sense, that usage pattern doesn't fit everyone.
For example, my mp3 player is mostly used in the car. This is somewhere where I'm unlikely to answer a phone call, and even if I did pressing the power button on the car stereo isn't much hassle (and exactly the same as if I'm listening to the radio or a CD I've not yet ripped).
Alternatively I might use it as for background music at a party (not letting drunk guests near the computer!). If my phone rings then I certainly don't want music pausing automatically, I want to be able to give lost late guests directions without pissing off the ones who've already arrived.
Then there are times when all I want with me is the phone because I'm somewhere where there is already music. On those occasions I'd much rather have a small dumb phone, not a relatively big smartphone.
There might even be times when I want music, but not to be interrupted by calls. Its much easier to switch your phone off and leave the mp3 player on when they're separate devices.