I think there's two possible options:
1) They really like him for some reason and strongly want him to be around for two seasons
2) They are having trouble getting other OFs to be interested in coming to Boston and this is the best they could do for that 4th OF type position.
I suppose there could be a 3rd option:
3) They know they are going to make a strong run at Josh Hamilton and wanted to get the other OF spot cemented prior to that push so they aren't left hanging.
There was a story on mlb.com that noted that industry insiders believe the Red Sox are going to make a run for Hamilton. I think that would be great, if the contract isn't Crawford-esque and it offers them flexibility to do other things as well.
In terms of comparable contracts, I think his best comps in terms of current performance and age would include players like Ryan Braun (5 years, 105m), Torii Hunter (5 years, 90m w/ Angels), Ichiro Suzuki (5 years, 90m w/ Mariners), and Matt Holliday (7 years, 120m).
Those are the big OF contracts of the past few years and they are players who have roughly the same impact that Hamilton had. Based on those players I think he can rightly ask for a minimum of 5 years. The Sox would have to really blow him out of the water in terms of salary to make the deal shorter, but they could possibly give him the infamous "opt out" clause that the Yankees used with CC and Soriano, which would give him the chance to make more money on the market in a few seasons and keep them from having to get into a huge deal in terms of length.