That's a truly horrible rate for purchasing miles (which is what you'd essentially be doing) in and of itself. IIRC the regular, non-bonused rate for directly purchasing miles from AA is something around 3 cents apiece which is still too high. But AA does occasionally run bonus specials on purchased miles that can bring the rate closer to a decent value if you need to top off for an award - but usually not for speculative purchasing.

If you were already planning to donate to a particular charity anyway and this would simply be icing on the cake, sure why not, but otherwise I've not found it to be worthwhile in the past. I also make sure to research any potential charity before I donate - IMHO Susan G. Komen is the last breast-cancer specific charity I would donate to.