I'm guessing someone at AA just said 6 months because it was a nice round number. Long enough that most people will have a chance to utilize the bonuses but not the entire year.

I'm also guessing that 95% of people will just continue their normal travel/spend patterns without putting any thought into strategizing the bonus timing. I'm sure some of us savvy FT'ers may play the game and postpone some large purchases until we trigger a bonus, but we're in the very (very) small minority.

And normal AA flight spend is not part of the 20%/30% bonus, so bonus timings shouldn't drive any behavior with regards to someone deciding to purchase a flight with AA or not.

If anything the bonuses will just drive more spend towards AA partners, which is exactly what they're designed to do. If I know I'm getting a 30% bonus on certain partners then I will be more likely to book that hotel through BAAH vs. going direct.