Major Leaguers make adjustments all the time. And you're forgetting that it's August. That .284 vs. lefties this year is over the course of 116 ABs, not just a month or two. If Bourn is only on a temporary high when it comes to hitting lefties, the temporary part hasn't shown signs of waning all 4 months of the season. That's not the best sample size, but it's not that small, either. It's big enough to at least entertain the idea of a professional making an adjustment, too.
A right-handed bat wasn't the only need the Braves had. Also, when Dan Uggla finally woke up, the need for that bat diminished, and the obvious holes in the OF and leadoff didn't. Of course the Braves wanted the prized bat in Beltran, but they didn't want to give up as much as the Phillies were willing to give up for TWO MONTHS of Beltran. It's not that complicated.
The only thing I don't like about this deal, obviously, was losing JS. He has all kinds of potential, but he's also been given all kinds of chances. I started to worry about him when he couldn't lock down a roster spot in Spring Training, and his health was continuing to be a problem. Bourn is a proven guy under team control another year, he'll play 140+ games/year, he's got the defense for the OF, and he secures the leadoff spot every day. That's an immediate upgrade on a team that's making a playoff push, and it did hardly anything to the farm system. I'll take it.