Test cases have little to no place in Agile development. They are the finest example of waste. I know that a lot of places still feel that more test cases equals better quality, that is clearly not the case. Quality is derived from process, which can involve test cases, but again test cases are very inefficient at catching and preventing bugs. I have evaluated multiple solutions and they are all lacking.
While I do advocate some Scenario based testing, which usually are a single line like "Can you place an order", traditional test cases are convoluted and difficult to maintain. Google advocated test tours and ACC (Attribute - Component - Capability) testing that is very efficient, but still can be difficult to maintain. I believe the product owners should maintain the ACC Matrix, but they are too busy dreaming up new ideas. My teams have been using Google Docs for 3-4 years to track scenario based testing, while not perfect it gets the job down, is lightweight, multiple people can be in there, and just gets the job done.
The time would be better spent creating repeatable automation, through unit tests or functional testing solutions like Selenium. I am hoping in the next year or so to move all testing away from manual and just use automation. Get automation plugged into a Continuous Integrated system, so it runs all the time.
Source: Quality Engineering Manager -- Test Early, Test Often and Test What Matters