Well the seal is mainly because of the AS-2 needing the right amount of air pressure inside your ear to give you the impactful bass and crisp and sweet trebles you'll grow to love about the AS-2's if you own them. Not sure how the ambient air effect this as its not entirely changing the amount of air behind the driver or anything, but somewhere in between where it might not have as much of an effect.
Since there is both a port to tune the bass and one for ambient sound I'm hoping that relatively little quality is lost when the ambient port is opened. However, like you I suspect that may not be the case. Either way, it's a good question to raise with Aurisonics.