Dunbar was a starting linebacker for half of the 2010 season and didn't perform that badly. The Saints are giving him another chance to come back and earn his job. However, Dunbar has also been an important special teams player, making his biggest impact in a Vikings game in 2008.
The Saints have proven over this entire off-season that they plan to improve the special teams so this could be a player they hope to keep to either improve defense or special teams. Either way, I like this move.