Measuring Quality in Agile

People keep wanting to measure defects in the agile process. It doesn't necessarily make sense to me how this relates to quality since the whole team participates in doing what is necessary to move small stories to done. It may measure productivity as fewer defects per story would mean that the team was being more productive. Anyways, agile is about collaboration and getting to done, so I don't think this is very relevant. For quality, measuring the number of issues found in production after release or tracking the over defects per system in production and seeing that track down over time would be better indicators of quality. Thoughts?