I thought Marco was ready last year, and I think his good spring has gotten others to see him as a near equal (or better) to Holt at 3B, SS and 2B. Holt's ability to play OF and 1B is something Marco cannot do, but with Shaw in the wings and back-up OF'ers costing a dime-a-dozen, I think we'd be fine, if not better, with Marco and Rutledge instead of Holt and Selsky.
Of course, if 2 IF'er or 2 OF'ers get hurt then we will be sorry we traded Holt, but those are chances we might want to take for the right return.
I see it this way, if many of us and Sox management value Holt so highly, there should be a desperate GM out there who also values him highly and needs a FT or near FT 2Bman, 3Bman, OF'er or a player that can maybe play 2B vs RHPs and 3B or OF vs LHPs and get 500+ PAs.