There are risks, too, though. I just pointed out in the Middlebrooks thread that he has a .400 BABIP and a 29 to 4 k/bb ratio. He's going to come back down to Earth, and he's likely to slump at least once before the season is over. He's not an elite bat yet, while Youk most certainly is. If the team wants to maximize its chances at making a title run this year, I think holding onto Youk is the best way to go.
When Pedroia gets back, that likely means moving Middlebrooks back to Pawtucket to keep him swinging the bat regularly, then calling him up just before September starts so that they can go into the playoffs with a roster of Youk, Aviles, Pedroia, Gonzalez, Salty/Shoppach, Crawford, Ellsbury, Ross/Sweeney and Papi at DH with Middlebrooks on the bench as a pinch hitter, defensive replacement, injury insurance, ect ect.
I love Middlebrooks, and I'm excited to watch him continue developing, but he's not going to keep up a .400 BABIP and when that drops, so will his overall line. He might be an elite bat in the future, but he's not there yet. Youkilis is.