Right now, today, of course, makes sense. Middlebrooks and Victorino are on the DL, and Ellsbury is nursing a groin pull. So this is about later when Ellsbury, Victorino, and Middlebrooks are back in business.
So the first thing not to forget is what happened last year when Youkilis was dumped to make room for Middlebrooks. After the Sox sent him to the White Sox and paid a big chunk of his salary, they badly needed not only a thirdbaseman to replace Middlebrooks on the DL, but also a firstbaseman to replace AGon who was traded, and even a DL to replace Ortiz, also on the DL. Nothing says Bradley and/or Iglesias is exempt from injury. If that happens, wouldn't it be nice to have Ellsbury and/or Drew?
Second thing is the Sox have played well with Ellsbury and Drew in the lineup because both are contributing. Ellsbury is second on the team in hits, and third in runs scored and total bases, plus he has 21 SB's. Drew has had one error at SS in 34 games, which is spectacular. Yes, Iglesias would get more chances, but there is real value in a SS who can be counted on to make the plays the should make. And his hitting seems to be turning around. This is a veteran team that really pulls for each other and plays hard. This might change if the FO dumps two veterans who were playing well and contributing to make way for two rookies, no matter how good.
Third thing is there should be no rush. It is now pretty obvious both Iglesias and Bradley will be starting next year--at SS and CF. Ellsbury and Drew are both on one year contracts and cost a lot more than Bradley or Iglesias. Better to keep both rookies playing every day either in Boston or at Pawtucket--and preparing to be regulars next year. If Ellsbury, Drew, or Middlebrooks is injured or badly injured, a capable replacement ready to go.
None of the above should be interpreted as in any way saying Iglesias and Bradley aren't already pretty darn good and a lot of fun to watch. Especially Iglesias, who is no doubt hitting over his head, but who also is playing 3B like he was born there. He deserves to play every day just for the sheer entertainment value. The Sox signed him for $10M when he was a teenager, and it sure looks like a good investment.
A small addendum as of June 5. Read Peter Abraham on extra bases today because he basically agrees with me. Having lots of good players--and not dumping them as bill-806 and softlaw and some others want to do--on a team is good, not bad. He also provides the UZR and OPS stats that show Drew is an above average SS on defense and offense.