Here's an academic treatise on loyalty, one in a series of reports from Cornell University's Hotel School.
"What we found is that the frequent guests became even more frequent after they joined the program, and the result was a 50-percent increase in revenue compared to those who did not join the program," said Voorhees, an associate professor at Michigan State University. "It's important to note that these guests are already some of the hotels' best customers, so the program is not only attracting the best subset of customers, but also fundamentally increasing their patronage too after they join."
Here is a summary and link to the report
Worth a read, for sure.