Tenet of XP: Customer Involvement
I actually really liked the idea of the customer being readily involved in the project. If the customer was easily accessible, any ideas/problems can be directly bounced off of them, guiding the project to a more successful and welcomed completion. This can be really hard though.
Let's face it; the customer sitting with the developer won't happen in some environments. I'm on a project where I have managers as the users, and so it is hard to get them in a meeting, let alone working with them directly. Sometimes, the environment causes this.
But, this approach is handy, and even if you aren't using an agile/XP approach, it is a good idea to regularly meet with the users and understand their needs.