I agree that you don't need to go out of your way for the highest possible sound quality while driving, but I can tell you with full confidence that even the stock iBuds put out far greater sound quality than my vehicle's sound system. I used to just burn CDs to listen to with my car's CD player, but I stopped doing that years ago because the speakers were just plain awful and the whole balance of the music being played is way off. My car speakers are about as resolving as a telephone, and I don't ever plan on spending many hundreds or even thousands of dollars upgrading them. What's more is that you don't need any bit concentration on the music to hear how bad they are.
So in that case, using my iPod and just about any set of headphones or IEMs is far more convenient and infinitely more enjoyable. As I believe I've stressed before in this thread, I listen to my IEMs only during my short daily commutes or while far out of town (and traffic). For longer drives, I typically play one of the few CDs I have that still sounds decent using my car stereo so that I don't ever have to fiddle with my iPod. I also hate driving downtown, and never listen to anything during such times.