I dunno. 700MB of mp3s lasts plenty long enough for me, and a CD in the stereo still seems more elegant than a device you plug in, or a stack of CDs in a changer in the boot.Because you can set the MP3 player somewhere it won't rattle, and it's not like you're going to trip over the cord. That, and the MP3 player can potentially hold, sort, and play much more music than every CD in a 6 disc changer combined. And it'll do all that while allowing you to skip to any song on the entire thing with a couple clicks. That said, there are also the bluetooth, SD, or Flash drive options that are pretty common in newer cars, as well.
The truth of it is, I don't use any portable mp3 player in the first place. Maybe I'm weird but that's kind of a big prerequisite for the USB/aux solution.