Well venting to the atmosphere is bad simply because the amount of air that the MAS reads going into the system will be quite different than the amount of air actually in the system because ait that would normally be recirculated is lost. This causes problems such as the car stalling between shifts and other bad idle/operating conditions. Some people do it and get away with it, but for most, it causes problems with the operation of the car under street conditions.

I'm sure Jeff can answer it better, but as I understand it, it is generally frowned upon in manual transmission cars, unless you are running a blow through MAFT setup.