XP2 is known to have an almost-infinite shoulder to the highlights. I have read at least one author who advocates XP2 as a one-film zone-system replacement. His method involved using 35mm XP2, bracketing extensively towards overexposure (relying on the shoulder of XP2 to control highlight contrast), and using variable contrast paper in the darkroom to manipulate contrast.
Other authors emphasize that XP2 can be exposed successfully as speeds from 100 to 800.
There's no doubt the shoulder of XP2 does make it a good tool for certain situations.