One nice aspect to buying miles is the ability to top off accounts unlikely to get to award levels. Or, with U.S.'s share miles with a kicker or AA's deals where you buy miles for another person but get miles in your account you can get the added bonus of essentially transferring miles to or from smaller accounts.

Just a quick example -- I currently need 4 one way tickets that would cost about $1,000. I can get them with 50k AA miles. Right now, with bonuses, I can buy 56k AA miles for $1105. I could buy the miles and use 50k of them for the trip, and I'd have 6k leftover miles for an extra $105. A decent deal, but ultimately probably not something I'd do since it's close to break even and I lose the 4xBIS miles by using reward tickets instead of cash tickets. But, the thing that makes it more attractive is that my wife has 52k miles in her account, which doesn't get much activity. I'd rather have those points in my account. So, if I buy the 56k miles into my account and use her miles to book the 4 tickets, it's essentially a mileage transfer from her to me. I have 56k more toward a million and we've essentially moved those miles into our main account where much more activity happens. Also, the change policy on the award tickets is slightly more generous, so that has value too. So, while I would not typically use 50k miles for $1,000 in tickets, buying miles I wouldn't ordinarily buy instead to book the reservation makes sense for us.