Signing up for credit cards to get airline bonuses and flying more are good options to earn miles. With an average redemption/valuation of 1 cent/mile or maybe 1.2 cents/mile why would it make sense to buy airline miles at 1.7 or 2 cents/mile?
Your experience might vary depending on the mileage program, but in most cases, buying extra miles is a waste of money.
Some legitimate uses:
- If you are close to a redemption value you want, there's no other reasonable way to get the miles, and you're able to buy the amount you need without going over too much.
- You have a lot of miles saved up, and your miles expire based on the last activity in the account. In this case, buying a small amount of miles can reset the expiry date for all the miles. Redeeming a small amount (e.g. a small charity donation) might also work in this case.
- Buying miles could help you maintain or upgrade your "status" in the airline. This might unlock enough discounts and free perks to be worth it in some cases.
Edit: Added more scenarios mentioned by Stian Yttervik and RobV where buying miles might make sense.
The most practical (though not necessarily cost effective) reason for buying miles is to reset your mileage expiry date.
Most programs have some some of expiry built into them so that if you don't earn/buy any miles for a certain period of time you forfeit your accrued miles. Buying miles will generally count as activity on your account and reset the expiry date. As buying the minimum amount of miles can be much cheaper than taking a flight, particularly with a long-haul airline, paying to keep your accrued miles can be worth it especially if you have a high balance.
As a concrete example me and my wife both have mileage with Virgin Atlantic. We used to fly frequently transatlantic as we lived in the US for work so racked up plenty of mileage. However after moving back home to the UK she rarely flies transatlantic anymore (and if she does it's usually by spending our accrued mileage balance from when we did fly frequently) so hasn't earned any miles in years. Virgin Atlantic mileage expires if you have no earning activity in 3 years and costs £30 (including transaction fees) to buy the minimum amount of miles (1000) so costs £0.30 per mile which is quite expensive BUT it's worth keeping the account active so that when we next want to take a big trip we still have our mileage balance to spend.
There are some rational reasons, I will provide one example. Please edit in if you have other good scenarios, we can make it a wiki answer.
Some airlines count purchased miles as regular miles (not "bonus" miles) - usually "bonus" miles do not count towards your status. I once bought a couple of thousand miles to keep my gold status in KLM's skyteam elite. When you qualify for a status you keep it for an entire year.
Lounge access for a year is worth the price of a couple miles. Having gold also means you can purchase certain seat options at half the price (or was it 75%?) which really matters on the long haul flights. Having that good seat makes the difference between business and economy flight only a glass of champagne. Avoiding that glass could save you or your company several thousand dollars.
So there are cases where it is a winning bet.