Over time, it will tear causing a vacuum leak on the motor. Porsche will only sell you the whole valve cover at a cost of over $600 and a few hours labor when you only need to replace this diaphragm.  
There are 2 versions of the 4.8L diaphragm. If your cap is closer to the front of the motor, it uses the 4.5L version (see picture labeled 3). If your cap is closer to the middle of the motor, it's the 4.8L version (see picture labeled 4). We've also learned that the 2003-2008 Cayenne Turbo and Turbo S will use the 4.8L version on the 4.5L engines regardless of where the cap is located.