It's a different safety net...

XP has a different safety net than traditional approaches.

Written spec -> functional tests
Written Design & Architecture -> refactoring & mentoring
Change Control -> Malleable code & visibility
Detailed writing ->oral communication
Everything thought out -> just in time
Look to the future and prepare -> look to the past and improve

Is this why combining Agile and Waterfall methods is the worst of all possible worlds? In both cases the safety nets are tightly woven (coupled?), one piece depends on all the others. Mixing and matching from both sides looses that "weaving".