I tend to imagine that the bulk of recent dreadnoughts are of the Overlord-class, seeing as the [LN] has four of them and three of miscellaneous classes. LABCs are of the Interdictor-class, and Siege Cruisers are either Archer- or Catapult-class (there's only one Falcon-class, and it's experimental). All the gunboats appear to be Defiant-class.
I always liked the class designations and think it adds a little bit more depth to RP when people piloting capital ships decide to explore classes within each ship type, it gives some lee-way into being able to RP your ship as having different sorts of modifications and such.
Like my [LN] Gunboat, the Lady Liberty, I call it a "New York Class" Hunter-Killer gunboat. To that end I play the ship as having advanced sensory equipment and more 'modern' computer systems in general than the average Gunboat.
But yeah, the classes of the dockable battleships are merely to add some extra depth and detail.