For IEM users, a big thing is microphonics (when you rub something, it conducts the noise to your ear). That trumps aesthetics for the most part.
Also, with an IEM, you'll hear less external environmental stuff, so you can hear differences if any better. But as always, it depends on the listener and external components. Some people can hear differences that are less obvious to others. Some users use external components w/ better drivers (e.g., Apple's original iPod raised the bar in portable music players way back when it was released) that aren't as affected by different cable impedances.