Abuse of congressional protection?

Our Constitution rightly contains protection for legislators against prosecution for performance of their congressional duties. Specifically, the Constitution says:

Section 6
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.

This provision was inserted by the founding fathers aware of how the monarchy they rebelled against had suppressed debate with heavy handed actions against parliment. However, this provision has been stretched and abused over the last couple centuries. What brings this up is the lawyer for former senator Mark Dayton (D-MN) cited this provision in attempting to get a former employee's discrimination lawsuit dismissed. The Supreme Court has agreed to hear the case which could make this a precedent setting ruling. Oddly enough, from the details contained in the article, I believe Dayton's legal team had a better chance of winning the case on its merits than by invoking congressional privilege. By heavy handedly attempting to dismiss the lawsuit they may end up exposing his congressional cohorts to a narrower interpretation of their congressional protection. This is one of those Supreme Court cases where the ruling could end up affected other parties much more than either the plaintiff or defendant.