Testers bring both specialized skills and a unique viewpoint that combine to enable the team to produce a better product more efficiently.
Testers provide checks and balances and are more efficient at finding defects, because they focus on it.
Testers find a different type of defect than programmers' unit tests do.
A dedicated tester role adopting test-first acceptance testing can help an XP team eradicate user-apparent defects just as it does unit-level defects.
If XP is like driving a car, the tester helps keep the team on the right road and makes them stop and ask for directions when necessary.
However, the tester role and testing functions must be integrated an isolated testing function in which testers are involved only at the end doesn't work.