One would say there are easier methods, that are more objective. Like: take the statistics provided by the app store on number of downloads, the price of the app (if not free), and from that you have the revenue.
The problem is that the app store does not provide any stats on the number of downloads. You can get a ranking, but that tells you nothing about download numbers. You can glean together bits of information from the occasional developer who says "my app that hit #4 had 15,000 downloads", but since no one (except Apple) knows exactly how the ranking algorithms work, even that's not accurate. Further, the number of downloads required to reach a given ranking changes daily based on the number of downloads on the whole store that day, and how many downloads a given app had on the previous day or two. Additionally, many of the top grossing games on the store use a freemium model, and that's even harder to account for, as IAP purchases don't affect rank at all (except for in the top grossing charts).
That's why this survey was important to do.