Effective detection is good; effective prevention is better

It's important to know when a defect occurs.  That's what inspection (aka testing) is for.  Inspection is non-value added activity but it is necessary to prevent leaking defects to your customer, whether that's the next process step or the end consumer.

Don't forget though that knowing when a defect occurs doesn't help you prevent defects.  For that you need to know how and why.

Agile practitioners will typically respond to a defect with "What kind of test can we write to catch these types of defects?".  This is good.  But it's better if we first ask the question "How can the system be designed such that these type of defects can't occur?"

Effective detection is good; effective prevention is better.