I'm gonna get some flak for this one but.....There are enough distractions riding a bike without adding more. You need to be 100% attuned to whats going on around you, the changing situations and be especially attuned to what that cage next to you is about to do...you know, the soccer mom on the cell phone, combing her hair and reading a book.....
Distractions on your bike like stereo or MP3 or Bluetooth Wireless add to your reaction time by distracting your thought process. You might need that 100th of a second sometime and if you don't have it, you might not be here to reply to this post.........
To each his own. For me, less distractions are better.