You also have to wonder just what sort of state the other ships are in. As we saw in Enterprise and even in TNG/DS9/Voyager, older models like the Daedalus class, and the Excelsior class, once they get going, Starfleet tends to pump them out, and even with upgrades every few years, those ships are still no contest to newer designs like the Constitution class or the Galaxy and Sovereign classes (of course the Sovereign was designed to battle the Borg). Who knows, we might see fleets of an upgraded version of the NX class (like the Columbia class).Makes me wonder how many ships they even have in the fleet. I got to imagine the number of ships they have is much smaller than you see in the 24th century so wouldn't that increase the odd's of seeing the same ships since you only got a select few to choose from for missions? Of course for me the most sense would be to have the Enterprise guardian Earth or Vulcan or some of your more important worlds.
Jason