Agile proposes people to make use of Index cards during requirement analysis and estimation phases. As per agile, it would help the development team and other stakeholders to change things quickly, ask questions without really "handing off" documents and waiting for things to happen.
Here are some of my thoughts on pros and cons of using 3*5 Index cards:
1. Very agile. You can modify the information with ease.
2. Sticking them to the wall keeps the information in front of your eyes all the time.
3. Easy to carry from one place to other.
1. Chances of getting lost are high
2. You may need a good secure storage space. Because over a period of time, the paper might become old and the chances of paper getting torn, and information getting lost is high.
One of the key advantage of using Spreadsheet(in parallel with Index cards) are
1. Quality of information would stay intact over a long period of time.
2. Sharing the information with onsite team located in a distributed team is easy
3. Spreadsheet is idea to keep track of information which keeps changing quite frequently.