Early in the development cycle, it’s perfectly acceptable to go for a policy of "zero bugs" and "100% tests". But as the deadline looms, these choices need to be reconsidered all the time and evaluated while keeping a close eye of the final goal. Very often, Agilists simply forget that their job is to produce software that satisfies customers, not software that meets some golden software engineering scale.
Worthy article written by someone who understands stuff. Read it.