I don't think any comparison is complete without some sort of salary survey.
However, Expatistan is not about finding cheap cities to work in. It's about comparing cost-of-living, independently of your income. Expats are usually not linked to average local salaries in any meaningful way, anyway.
I explain it much better in the website: http://www.expatistan.com/faq#average-salary
For instance Paris is described to be about as expansive as San Francisco. But in Paris, you have your kids' school and your health insurance for free (well, included into your taxes). Plus, if you studied there, you don't have a student loan to pay back.
Or simply give salaries net of taxes. In many countries, a large chunk of the taxes are paid by enterprises, and never appear on your salary sheets, although they're part of your compensation package (they're paid by your employer, because he has to in order to legally hire your workforce, and they serve to fund services you'll benefit from).
E.g. if you compare median salaries in San Francisco and Tokyo, you might get similar numbers. But if you're an ESL teacher, you might get paid quite a bit more in Tokyo, whereas if you're a software engineer you'll get paid a lot more in SF.
In particular, this data can be immensely useful if you live in X with a known salary and you have an offer for a different salary in Y, and you're wondering how it'll stack up. This will answer that question for you.