Role of Usability in the agile process

Hi,

in the last weeks we often have the discussion @ TT which role should the usability engineer have in the agile/scrum process. Is the usability engineer a member of the team or on the same level as the product owner?

In the article "Twelve emerging best practices for adding UX work to agile development" on the blog "AgileProductdesign.com" Jeff Patton advises as a main point that the UX practitioners should be part of the customer and product owner team. This has the advantage that the UX expert is in the position to decide and influence what should be build in the next sprint.

In my opinion it's not always clear how this can be handeld during the sprints to keep up the communication between the development team and the UX team - also to get fast feedback and not wait until the end of a sprint.
Another interesting question is how efforts/estimation should be handled? If the UX team is part of the product owner team and not part of the "TEAM" the efforts will not be part of the estimation at the beginning of the sprints. I'm not sure if this is good. Jakob Nielson also mentioned in his blog-article "Agile Development Projects and Usability" that story points should be assigned for the interaction design and usability.

Do you have any opinions and suggestions on this?

Br, Claudia