Nothing wrong with this move. Not sure why anyone is puzzled by it.
Simply put, if injuries occur down the stretch or into the playoffs, the Bruins are better off with Holden as the 4th LD spare than they would be with O'Gara.
Holden is a 3rd pair D on most teams, he's got almost 400 NHL games under his belt, has averaged 20 mins a game for his career, 19 this season.
With the emergence of Gryz, and with Zboril and Lauzon soon to be pushing down in Providence, soon to be 25-year old Rob O'Gara didn't appear to have much of a future in Boston.
A 3rd is a little bit expensive, but we don't know what the market is like for depth LD. Washington gave up a 3rd for Kempny yesterday, so that appears to be the price for 3rd pair/spare LD.
Given that the UFA rental LD market is pretty darn thin this year (Your looking at older guys like Seidenberg, Gorges, and Oduya), in theory would of raised the price on these types of players. I'd argue of the UFA Rental depth LD, the Bruins might of gotten the best one.