Loyalty programs are a source of profit for airlines exactly because of this. They sell millions of miles. (although most to credit card companies and probably they will get better rates.)

For customers it only makes sense to buy miles to top up an account if you require few extra miles for an award. The opposite of what you are trying to do. You should already have 75K miles and buy 15K extra.

Or it might make sense if you can profit from a promotion. (eg. the 100% extra miles promotion running on US Airways and ending today). You will see that airlines run promotions several times a year, you would almost be crazy to buy miles at full price.