I included the spine as a visual reference to the Eagle.
I did toy with the idea of not including one, but the model didn't look right without it.
The central section containing the port and starboard docking connectors / doors descends in my mind's eye - a self contained airlock for use on a planet's surface - so the spine would perhaps aid stability in this instance.
Although the design of the Kestrel doesn't require a connecting spine from a structual point of view, neither did the Hawk, which also featured one.