XP relies on a "customer" to decide which features should be implemented
and when. A "customer" may be a product/project manager, customer service
employee, a marketing resource, etc. or even a user.
The tragedy here is that this "customer" provides direct, face-to-face
input with the development team to determine which features must be
implemented and when.
As interaction designers and user-centered types, we know that users or XP
"customers" can't articulate what they want, so their direct input can
result in serious interaction problems (among other issues).
The XP methodology was crafted by developers, so this might explain why
this flaw exists, however I feel that it could be easily modified to
properly address the human factor in software.