With respect to the valuation of the offer, it looks quite favourable. Customer Service may tilt it against.

Let's take our $10,000 balance, and compare. You can choose $550 (fully taxable) or $466 + 6000 miles (miles not taxable?). Opportunity cost for the miles is therefore the $84*(1-your marginal tax rate) for 6000 miles. That means you're paying at most 1.4 cents per mile, and more likely 1.1 cents per mile.

Of course, I would request that you not ask taxpayers to subsidize your miles.