If you hang out with agilists for long, you'll inevitably hear accusations that some team or other is 'not really agile' because they do not precisely follow the established practices--scrum, XP, etc. I've criticized this thinking several times on my Agile Testing blog.
Here at Borland we have an agile process evangelist, Dale Schumacher, who works with the various scrum teams. Whenever a team that he's working with considers changing an agile practice, Dale grills them on it. I used to describe Dale's questioning this way: Dale's job is to represent the ideal of the practice; if the team in question is going to deviate from that ideal, Dale is there to make sure they understand the ramifications of the deviation.
As my understanding of agile changes (matures, I hope), I would describe his questioning this way: Dale's job is not to represent the practice, but to represent the agile principle(s) behind the established practice. If the team is going to change the practice, Dale is there to make sure that the team understands whether and how the changed practice still fulfills agile principles.