How they do in a mid season series against the Braves proves nothing. I seriously doubt the Brewers would send out Teheran, Rea and Hauser as their starters in a playoff series. They just got done playing six series against winning teams and won four of them. The fact they have a winning record, and have had one all season long, is because they've been good enough to be the playoff contender they have been so far.
I also don't get the "If it wasn't for the division they play in they'd be sellers" comment. I'm not directing it at you personally since it's been a standard comment all over. If the division is so inept why do they have the same number of teams who'd make the playoffs if the season ended today as the other two divisions? Why do they have the same number of teams with a winning record as the other two if it's so much worse than the others? Obviously that can't be blamed on an unbalanced schedule. While it may not be the best division, it's not so far behind that it's a lock only one team will advance. Given the fact the Brewers are the number two wild card team today kind of shows maybe there's more than one route to the playoffs for the NL Central division teams. Or perhaps it shows there aren't a whole lot of great teams in any division. Either way the Brewers position as a playoff contender is far from simply being in a weak division.
All that said, I agree we shouldn't trade the farm for a one season run. Not because they don't stand a chance in the playoffs but because there's enough in the system and what trades would give us in return to think this team can be a contender for the foreseeable future. I'm not a fan of the all in mantra.