Phones with mp3 players are becoming more and more common every day. Would you think mp3 on phones would be restricted to just a few mp3 ring tones? Of course not. People expect technology to steadily add new features to their devices having phones with mp3 players was just the next logical step.
Having phones with mp3 players made phone makers aware of a up till then inexistent issue: memory capacity! If a few megabytes were more than enough to store thousands of contact records and sms, having mp3 songs in there is a completely different matter. When it comes to mp3, there is no such thing as too much space.That's why you now have phones with up to 90Mb internal memory, and most offer expansion slots where you can add more memory via small memory cards.
Using your phone to listen to mp3 songs is not only convenient, it's plain logical: you carry your cell phone around most of the time, the same for your mp3 player why not have both in a single package? It won't take long till all cell phones will become phones with mp3 players it's so useful that its just a matter of time.