Amazing how a car enthusiast based internet forum, has so many people who have little or no knowledge in relation to cars and modifications.
The fact does remain that all vehicles must comply to national vehicle standards, that includes modified vehicles. ADR's ring a bell
This is where the blow off valve situation comes into play. (obviously relates to later model engines, check the dotars site for relevent ADR's)
BOV's vented to the atmosphere on vehicles which have these ADR's are illegal, and you will be fined.
It goes against two ADR's listed below. Just to note further, any air that enters the engine through its induction system ie(turbocharger, airbox) must exit through the exhaust. If you want further details on this, gee I dunno contact an engineer. (For the record, I am an engineer)
Not being plumbed back has the following issues
1. It vents crankcase gasses to the atmosphere - Against ADR 79
2. Increases the noise level/induction level - Against ADR
I think people should start doing some research before assuming their modifications are legal, then starting topics on internet forums crying about their defects. Wether they like it or not, most of the time the police are certainly within their right to give you defects. So remove that possibility by making your vehicle legal, might be a good start
You can mod your car correctly, and follow the guidelines which are there for you, get 0 defects, or go ahead and think you can outsmart the law. I know which path I will follow.