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".