I think that Willie Harris is a bit too overrated. He has some value as a bench player because of his speed but the fact that he can only truly play 2B makes him less valuable. He refused to learn how to play SS and he isn't very good in CF. So he's not really a utility-man, he's really just a speedy backup 2B with absolutely no power. I think there's a reason Willie had to sign a minor league contract.
I agree that Mirabelli for Loretta was a really good trade for Boston though.
Alex Cora's glove makes him more valuable to a team as a utility-man. He can play 2B and SS. Willie plays one position. I wouldn't even be shocked to see Willie not even make Boston's roster since they already have Loretta, Graffanino, and Cora to play 2B. At that point, Willie has to beat out Adam Stern for the 4th OF / backup CF job. I'm not even sure he will be able to do that. Willie should have signed with the Rockies. He would have had a better chance of making their roster and having a good offensive season. He would have actually been able to platoon some with Luis A. Gonzalez at 2B with the Rockies.