If you're boosted, it's best to keep that thing disconnected as well, being as you don't want to pressurize your head/crank case.

However, if one of these isn't needed, I don't see why you would want one.

The main purpose of this is to eliminate the return system to keep the intake air cleaner. After 50K miles this makes your TB and IM look like shit on a stick.

Im ghetto, and didnt want to spend the few bucks on one, and just left mine unfiltered. I dont give a shit about the atmosphere. My residue can float around all it wants.