Having a market hub that doesn't involve climbing out of a gravity well would certainly make sense from a perspective of space-economics, especially since most of our ships probably handle atmospheric flight about as well as the average brick.
They probably wouldn't be useful as a planetary defense platform themselves, since a planet in space could be attacked from any direction and they would thus be trivial to bypass, but they would be necessary as a servicing station for defense fleets full of non-atmospheric ships and would probably also serve as something of a fortified harbor against attacks at stellar operations.