It's not as simple as the OP and others think.
For one thing, Drew has hit the ball better than Middlebrooks and fielded a lot better, so the odd man out should be Middlebrooks and not Drew. The problem with that is Middlebrooks is the heir apparent to 3B and needs to play. Sending him back to Pawtucket doesn't work because Ciriaco is gone and the Sox need four guys to man 3B, SS, and 2B and have a spare.
Iglesias needs to play every day because he is the heir apparent at SS. Pedroia already plays every day because he is the one and only secondbaseman.
Drew should play every day because he is fielding very well--certainly much better than Middlebrooks with his team leading 9 errors--and is a lefty bat in a mostly righty pitcher universe. He just hasn't delivered yet. Plus he has seniority and is being paid a lot. And, oh yes, the Sox are winning with Drew at SS.
Ciriaco was let go because these four are the four best the Sox have right now for these three positions, but no one among them wants to be or even should be the utility infielder. Iglesias has the job because he is the junior guy, because he is now on the big club for good, which should make him happy, and because he is far and away the most versatile of the four.
And let's not forget 2012 when exactly the same comparisons were made between Youk and Middlebrooks that are now being made between Iglesias and Drew. Exactly the same comparisons. So the Sox dumped Youk, and not two months later badly needed a thirdbaseman who could hit and a DH who could hit, but Youk was by then playing for the White Sox and being paid by the Red Sox. Not something one would want to repeat. The many advocates for benching Drew have clearly forgotten that's what happened to Youk last year--Middlebrooks had 3B locked down, Ortiz was the DH, and AGon was 1B. A benched Youk was an unhappy Youk even though the other three players--Ortiz, AGon, and Middlebrooks--had fully demonstrated they were each better than Youk. Plus don't forget that trading Drew away will mean the Sox have to go find another utility infielder.
Let's also not forget that, while Iglesias no doubt would love to play everyday, his biggest demand all along is to not be sent back to Pawtucket where he has struggled.