Ted O’Grady has an interesting post on the customer and their role on XP teams. I agree with what he has said.
It’s also important to think of the context that the software is being used in. I learned a lesson about getting users to test our software in our office vs. getting them to use the software in their own office. The Hawthorne Effect seemed to really kick in when they were on-site with us. When we observed them using the software in their own business context to solve real-world problems, they used it differently.
If the customer is out of their regular context, that may have an effect on their performance on an agile team. Especially if they feel intimidated by a team of techies who outnumber them. When they are in their own office, and their team outnumbers the techies, they might be much more candid with constructive criticism. Just having a customer on-site doing the work with the team doesn’t guarantee they will approve of the final product. Often I think there is a danger they will approve of what the team thinks the final product should be. When we had rotating customer representatives on a team, groupthink was greatly reduced.