With rosters expanded and Indianapolis' season now over, the promotions of Diaz, Karstens and Veal were no-brainers. Diaz played fairly well in an extended stint with the Pirates earlier this season and only got sent down in a roster crunch. Karstens pitched very well in Indy and was just there to rehab a back injury anyway. And Veal hopefully won't pitch much with the Pirates, but he's a Rule 5 pick.
Bixler's just a disaster and I wouldn't be upset if he never played for the Pirates again, but here he is. Hacker isn't very good, but at least he can start and therefore has the requisite smidgen of upside that all mediocre starting pitching prospects have until they're tried in the bullpen. I'd bet on major league hitters tearing him apart, but with several batting practice pitchers in the bullpen already (Chris Bootcheck, Steven Jackson, Virgil Vasquez, Denny Bautista, Phil Dumatrait), I suppose it can't hurt to give Hacker a handful of innings.