Once DS9 got the Defiant....things got interesting. 
I realize this is a thread about Hard Star Trek, but a thought occurs to me: (which isn't often 

 )
The Dominion War is over.  What is to be done with the Defiant class  line of ships?  Are they to be repurposed for more scientific endeavors?   Is the class to be decommissioned? Mothballed?  Used as training  vessels?
If I were to do a story set in the Prime universe (DS9 post-Dominion  War), I would have some of the Defiant class retasked as patrol ships,  allowing larger cruisers to do their primary functions: exploration and  scientific operations, diplomacy, humanitarian aid, etc...  A couple of  Defiant class ships could be repurposed as training vessels.
For training, here's what I would do:
Runabouts-- Essentially small starships in their own right.  (They even  have individual NCC registries.)  For cadets about to begin the space  exploration part of the curriculum, a few cadets assigned to a runabout,  with rotation to allow all cadets on board to handle  command/helm-navigation/scientific application/communications/small  level tactical operations.  For a couple of weeks, one cadet is in  command (under the supervision of a commissioned Starfleet Officer),  while the others take on the subordinate functions.  After those two  weeks, the cadets rotate their positions.  Another cadet is now in  command. 
During their final year, a crew of cadets (supervised by a small crew of  commissioned SF officers) would train aboard a Defiant class.  (Yes, I  know there was an episode where cadets were training on, and then took  command of a Defiant class ship 

  ).  The Defiant class is small, but it can allow the cadets to handle a  wider range of training tasks.  Also, the spartan accommodations aboard  ship would make for (as we called it in the Army) "good  training".