Aerodynamics are often more important on a road bike. Remember that a roadie thinks 10 percent is a steep climb while a mountain biker won't even be in his granny gear for that. Aerobars are an even more extreme example, and they are used on very short rides where oxygen debt isn't as big of an issue. If you look at ironmans or ultra endurance events, you'll see they don't spend the whole time in the aero position, and they often have a wider stance than aerobars for shorter events.

Also, people seem to be of the opinion that as long as your hands are as far apart as your shoulders, breathing is as free as it gets. I can't verify that, but sure, why not?