There is little understanding of the concept of Parliamentary privilege outside Westminster. The term tends to arouse suspicion: privilege is against the spirit of the times – it smacks of unfairness and elitism. It did not help when some of the MPs prosecuted for fiddling their expenses a couple of years ago tried to base their defence on the doctrine of privilege (the Supreme Court gave that argument very short shrift  ). So why should MPs be exempted from laws which apply to everybody else?
The most important aspect of Parliamentary privilege is freedom of speech in Parliament which derives from the 1689 Bill of Rights. It is a right which can be justified on simple functional grounds: MPs cannot do their job unless they can speak freely without fear of being prosecuted or dragged through the courts by wealthy litigants. For that reason it is essential to maintain Parliamentary privilege, something that is recognised in all modern, democratic constitutional systems.