Ditto what all the previous posters have said.
Unless you can book your flight right now, simultaneously with when you are purchasing the extra miles, I would probably not take advantage of this offer.
If you are ready to book for a time in 2016 for which the airline has flights available, you could go ahead and call, saying you want to use Advantage Miles for that booking (yes it will cost you a little to have a rep book the ticket vs. doing yourself on line, but would be worth the fee in this case). Then WHILE THE REP is on the line and can verify that you can actually use the miles for the flight you want, then buy the miles on the spot.
But, there is no way I would personally buy the miles then wait until 2016 to book a flight......for all the reasons everyone above mentioned.
It is actually sometimes easier to book a coach ticket and use miles to upgrade to business, than to just book a coach ticket using miles. Even then, it can be hard to upgrade on certain popular flights/routes. And, I'm sure you know there will still be fees associated with the ticketing just to use your miles. For a recent trip, we had to pay $175 each way ($350 per person) and then the $25 or $35 agent assistance fee (but it was so worth it when booking for two people using FF miles...just to make sure you actually get on the same plane).
And, while I am not in charge of the AA miles program, we seem to get special offers to buy miles "all the time" from the various airlines.
Another way to log up a lot of miles is to do your shopping thru them. They offer a bunch of on-line merchants....my sister and her family rack up gobs of miles that way. So, if you buy the LandsEnd or some other merchant, just click on the AA shopping program, which takes you to that site, and then you rack up miles for all you buy. That's also a good way to keep activity on your FF account, so miles don't expire from lack of activity, if you have miles at several airlines.