1. I can see benefits for both primary and secondary data. If you collect it yourself, then you have complete control over the methodology and will probably have significant confidence in the outcomes. You were in control of the research, so there is only one chef in the kitchen. I would think this would also increase the validity of the findings as well.
However, using secondary data also has benefits. Sure, you are reading someone else's findings so the chances of there being some kind of error is present. But, one thing all good scientists are always looking for is was to improve upon the experiment. Every empirical article I have ever read has an "implications for the future" section. By using some else's data, you have the ability to deeply analyze what they did, find the holes, and ...
This response discusses the advantages of using primary data sources versus secondary data sources. It also gives an opinion about the educational process of grading on a curve.