Whether it is Morrison or Martinez, you have the same roster issues. That Martinez can play the OF poorly really doesn't add to his versatility. He's really not a defensive wizard out there. "Defensively mediocre" would be better goal for him.
Sure they might have less roster flexibility. But it's not like we are bandying about some great depth chart names for the bench. So someone gets cut or demoted from the set of Leon, Swihart, Brentz, Holt and Marrero (assuming Hernandez starts at 2B, which is not a given to begin with.)
The Sox could bring in Morrison over Martinez, spend the difference in cash on a pitcher, and the 4 man bench would have to come from that same set of Leon, Swihart, Holt, Marrero, and Brentz. Only Holt and Hernandez have options left.
To me, I demote one of the guys with options and cut Brentz. 25-man roster set. Not really less flexible than if we did have Martinez.