The challenge is that an airline would like to base their rewards on what matters to them, which is dollars spent, but that makes the program a sitting duck for the IRS. The IRS would love to make this taxable income in the case where the company pays for the ticket but the individual gets the award -- obviously a common situation.

So it's necessary to have a proxy for dollars spent that gives the IRS problems. Miles works, and so would segments, but clearly both can be gamed. Still, I think you'll have to pick one and be stuck with a system that is an imperfect proxy, unless you want to bring in Big Brother IRS which will doom not only your program, but maybe all of them.